Add 'expected' OMP stderr messages to test exclusions

Running clang builds generates stderr messages like

OMP: Warning #96: Cannot form a team with 64 threads, using 1 instead.
OMP: Hint Consider unsetting KMP_DEVICE_THREAD_LIMIT (KMP_ALL_THREADS), KMP_TEAMS_THREAD_LIMIT, and OMP_THREAD_LIMIT (if any are set).

These are due to the fact we use OMP_THREAD_LIMIT=1 to limit OMP to one thread, and are unavoidable (no way to suppress them).

This MR adds them to the exclusion line skipper in LHCbExclusions.py