generic/500: skip this test if formatting fails

generic/500: skip this test if formatting fails

This testcase exercises what happens when we race a filesystem
perforing discard operations against a thin provisioning device that has
run out of space.  To constrain runtime, it creates a 128M thinp volume
and formats it.

However, if that initial format fails because (say) the 128M volume is
too small, then the test fails.  This is really a case of test
preconditions not being satisfied, so let's make the test _notrun when
this happens.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>