add heartbeat to TCT tests comparing many entries
ART grid jobs for some of the TCT tests (test_bulkProcessing_*) are regularly being killed by the pilot job due to apparent inactivity, which occurs because the diff-root comparison can last for a while and does not print anything to the log file until completed. With the changes from this MR, a dummy file will be regularly touched in parallel to hopefully keep the job alive for 5 hours regardless. This is likely temporary, until we get a better idea of the time taken by those comparisons (there is some variability) and maybe find a better way to address the problem --- reducing the number of compared entries is not a good idea since it would render the tests less sensitive to FT0Vs.
In addition the number of entries for the comparison to the fixed reference (instead of previous nightly) is reduced to 10 for the time being.