So I tried the following experiment.

I create a class AUTOTEST and made it the root class. It looks like this:

note
	description: "Summary description for {AUTOTEST}."
	author: ""
	date: "$Date$"
	revision: "$Revision$"

class
	AUTOTEST

create
	make

feature {NONE} -- Initialization

	make
			-- Initialization for `Current'.
		do
			-- nothing to do
		end

feature -- Classes under test

	gmp_integer: detachable GMP_INTEGER

	gmp_rational: detachable GMP_RATIONAL

	gmp_float: detachable GMP_FLOAT

end


I then opened GMP_INTEGER, GMP_FLOAT and GMP_RATIONAL in the editor, went through the generate open classes dialog, and it works (that is, it generates some tests, some pass, some fail, some are invalid).

I still can't see the Groups pane though, so I can't open additional classes that I want to test (remaining problem one).
And having to create a special class for a root is a real pain (remaining problem two)
And I'm also interested in extracted classes. How am I supposed to get that to work, if I need a special root class? (remaining problem 3).

This Autotest stuff needs a lot of work doing to it.

Remaining problem 4 - the tests don't compile. Messages such as:

VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_061.generated_test_1 (test)	39, 35	
Error code: VUAR(2)

Type error: non-compatible actual argument in feature call. 
What to do: make sure that type of actual argument is compatible with
  the type of corresponding formal argument. 

Class: NEW_TEST_SET_061
Feature: generated_test_1
Called feature: minus alias "-" (other: [like Current] attached GMP_RATIONAL): [like Current] attached GMP_RATIONAL from GMP_RATIONAL
Argument name: other
Argument position: 1
Formal argument type: attached GMP_RATIONAL
Actual argument type: NONE
Line: 39
        end
->      execute_safe (agent v_6.minus (Void))
        v_7 := last_object			
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_061.generated_test_1 (test)	43, 56	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_061.generated_test_1 (test)	47, 43	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_061.generated_test_1 (test)	61, 37	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_060.generated_test_1 (test)	42, 34	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_060.generated_test_1 (test)	83, 47	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_059.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_057.generated_test_1 (test)	51, 43	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_055.generated_test_1 (test)	45, 43	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_054.generated_test_1 (test)	73, 47	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_049.generated_test_1 (test)	52, 47	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_048.generated_test_1 (test)	42, 46	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_046.generated_test_1 (test)	34, 45	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_046.generated_test_1 (test)	49, 46	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_045.generated_test_1 (test)	34, 45	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_044.generated_test_1 (test)	39, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_043.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_042.generated_test_1 (test)	42, 43	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_041.generated_test_1 (test)	41, 54	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_039.generated_test_1 (test)	33, 38	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_038.generated_test_1 (test)	38, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_037.generated_test_1 (test)	48, 41	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_036.generated_test_1 (test)	91, 48	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_034.generated_test_1 (test)	49, 39	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_031.generated_test_1 (test)	30, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_030.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_029.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_028.generated_test_1 (test)	35, 56	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_026.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_025.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_023.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_021.generated_test_1 (test)	74, 51	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_021.generated_test_1 (test)	85, 57	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_020.generated_test_1 (test)	30, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_017.generated_test_1 (test)	62, 54	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_016.generated_test_1 (test)	60, 44	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_013.generated_test_1 (test)	38, 50	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_013.generated_test_1 (test)	40, 40	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_013.generated_test_1 (test)	50, 15	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_012.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_010.generated_test_1 (test)	38, 35	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_009.generated_test_1 (test)	59, 45	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_009.generated_test_1 (test)	65, 52	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_008.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_007.generated_test_1 (test)	44, 56	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_006.generated_test_1 (test)	35, 48	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_005.generated_test_1 (test)	60, 45	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_004.generated_test_1 (test)	34, 41	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_003.generated_test_1 (test)	28, 10	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_002.generated_test_1 (test)	39, 40	
VUAR(2): Non-compatible actual argument in feature call.	NEW_TEST_SET_001.generated_test_1 (test)	28, 10	
Obsolete Call: Call uses obsolete feature. Call to feature `empty_operands': This function will be removed as non-void-safe. [22.07.2013]	EQA_GENERATED_TEST_SET.execute_safe (testing)		
Unused_local_warning: Unreferenced local variable(s) `l_empty'	EQA_GENERATED_TEST_SET.execute_safe (testing)		
Obsolete Call: Call uses obsolete feature. Call to feature `make_from_c_pointer': Use `make_from_c' instead.	NEW_TEST_SET_019.generated_test_1 (test)