ftests: Add a simple cgclear functional test

ftests: Add a simple cgclear functional test

Add a test that invokes cgclear.  Note that this test is especially
destructive and is skipped by default.  Github Actions invokes it
separately as the final step.

-----------------------------------------------------------------
Test Results:
	Run Date:                          Mar 23 16:07:55
	Passed:                                  1 test(s)
	Skipped:                                 0 test(s)
	Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
	Test                                    Time (sec)
	---------------------------------------------------------
	setup                                         0.00
	028-cgclear-basic_cgclear.py                  1.80
	teardown                                      0.00
	---------------------------------------------------------
	Total Run Time                                1.80

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>