DerivationFrameworkTrainsART: Add lightweight athena reading checks to PHYS+PHYSLITE data18 and mc20 tests
This is a follow-up to !60047 (merged). Here we add an additional check to the PHYS+PHYSLITE data18 and mc20 ART tests. These new checks basically highlight shortcomings in reading these files in athena, which are not always caught in vanilla ROOT validation checks.
I also updated the data18 tests such that we now run 4 workers instead of 8. This stems from the fact that the input has only 6k events, which is not enough to fully test the new SharedWriter with that many workers.