Without this patch, the openmp project's test suites do not appear to
have support for negative tests. However, D78170 needs to add a test
that an expected runtime failure occurs.
This patch makes not visible in all of the openmp project's test
suites. In all but libomptarget/test, it should be possible for a
test author to insert not before a use of the lit substitution for
running a test program. In libomptarget/test, that substitution is
target-specific, and its value is echo when the target is not
available. In that case, inserting not before a lit substitution
would expect an echo fail, so this patch instead defines a separate
lit substitution for expected runtime fails.