tests/pc8: Update to latest igt infrastructure

tests/pc8: Update to latest igt infrastructure

- Use igt_require for magic SKIP results while still printing results
  for all subtests.

- Wrap setup/teardown code outside of subtests with igt_fixture. This
  will ensure that the so wrapped code isn't run when an
  igt_require/igt_assert failed (and we're essentially just printing
  the SKIP/FAIL result for all subtests) or when just enumerating
  subtests.

- Use igt_exit. Otherwise SKIP/FAIL results of subtests won't be propagated
  correctly.

- igt_subtest checks should be run unconditionally to make sure we
  always enumerate the same set of subtests. A bit on the line here
  since this is a subtest hidden behind a runtime flag.

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