Replace PerfMon with PerfMonMT in Trigger ART tests
Nearly all Trigger ART test enable PerfMon by default. For athena.py and Reco_tf.py tests this used to be done with the --perfmon command line argument. Replace this with --pmon=perfmonmt to change from the old PerfMonSvc to the new PerfMonMTSvc. This looks to work neatly for the use cases in question.
For athenaHLT.py this is done by including the JO snippet TrigCommon/PerfMon.py. Update this snippet to include PerfMonMTSvc instead of the old PerfMonSvc. Thus far, neither of the two services is actually useful for the athenaHLT workflow, but at least switching to the MT service brings us closer to what we'd potentially want to have working in the future (soon?).