lib/tests: make sure igt_skip in igt_fork is forbidden

Another corner case to check.

v2: Rebase. Note that we still have the SIG + 128 exit code, that's
how igt_waitchildren forwards child death to the parent's exit code.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>