[OpenMP][NFC] Fix `not` sustitution in tests

D78566 introduced a `\bnot\b` lit substitution in OpenMP test suites.
However, that would corrupt a command like
`FileCheck -implicit-check-not` or any file name like `%t.not`.  We
could use lookbehind/lookahead assertions to avoid such cases, but
this patch switches to `%not` (suggested during the D78566 review) as
a safer option.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D79529