Add testcases for css change flags

Add testcases for css change flags

Add various tests for the change flag computation that
we do in the css selector tree.

test1: Just test the basic machinery of this test
test2: Trigger every change flag at least once
test3: Test that multiple states combine as expected
test4: Test negations (known to produce wrong results)
test5: Test a complex selector (not producing the expected
       output atm)
widget-factory.ui:
       The real thing: widget-factory+Adwaita. Note that
       this expedts to be run with GSETTINGS_BACKEND=memory

Note that test4 checks the wrong results that we currently
produce for selectors involving :not. It will have to be
updated when we fix the handling of :not. The widget-factory.ui
testcase will certainly also be affected.