[OpenMP] Fix check-openmp after r354553

Calling add_openmp_testsuite will add the tests to check-openmp unless
EXCLUDE_FROM_ALL is set. This is problematic because the tests for OMPT
will be included twice which doesn't work if the same test is executed
concurrently by multiple threads.

See:
http://lab.llvm.org:8011/builders/openmp-gcc-x86_64-linux-debian/builds/163
http://lab.llvm.org:8011/builders/openmp-clang-x86_64-linux-debian/builds/184

http://lab.llvm.org:8011/builders/openmp-clang-ppc64le-linux-rhel/builds/133
(On PPC some failures are unrelated to r354553, the bot has been red before
and this commit is not expected to fix that. For a proper patch please see
https://reviews.llvm.org/D56286.)

llvm-svn: 354572