[OpenMP] Add "not" to test dependencies.

The `not` program is used to test executions prefixed with `%libomptarget-run-fail-`. Currently `not` is not used for libomp tests, but might be used in the future and its dependency does not add any additional burden over the already established `FileCheck` dependency.

Required to add libomptarget testing to the Phabricator pre-merge check (see https://github.com/google/llvm-premerge-checks/issues/368)

Reviewed By: jdenny, JonChesterfield

Differential Revision: https://reviews.llvm.org/D115454