Create reproducible ROOT files and clean up tests
- Make sure that ROOT files have reproducible content by telling ROOT not to put timestamps in the file (look for "reproducible" in https://root.cern.ch/doc/master/classTFile.html).
- The caveat is that the filename and the FID (GUID/UUID) are explicitly put in the file by Gaudi. Usually Gaudi would generate the FID from the current time, making the contents not reproducible. However, if an XML file catalog is prepared before running, the FID can be controlled.
- Small test-related cleanups
- Reproducible event times in tests (don't use current time, needed to test the output reproducibility)
- Show exception traceback in test
- Do not gitignore ref files
- Speed up histoex tests