Use empty stderr reference in all tests
Make sure all tests define an empty stderr reference log.
This forces the preprocessing of stderr with the preprocessor passed to
validateWithReference
and thus known messages there are ignored, such as
OMP: Warning #96: Cannot form a team with 8 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).
Addresses residual clang test failure, e.g. at
https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/build/1013/
and 'future proofs' the remaining tests.
Edited by Rosen Matev