As the hang injection now itself checks for validity before use, the
tests don't need to do so themselves. Except in certain situations! If
the test forks, it should do requirement checks before the fork (so that
we don't anger the igt gods) and if the test plays around i915.reset
then it needs to do an early igt_require_hang_ring() that is not
affected by the changes to i915.reset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>