Useful for creating common setup code in igt_fixture which is only
needed by a subset of tests. And since I'm a good citizen it comes
with a library testcase/example included.

v2: Make testcase nastier to ensure a subtest group SKIPS when it's
parent is skipping already. I accidentally got this right, but let's
make sure.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>