Crash in qm tests with checking of TTree output with too long branch names

During qm tests with TTree output, the output checker crashes when branch names are too long (shifted to another line) and subsequent indexing in the lines doesn't apply as given in https://gitlab.cern.ch/gaudi/Gaudi/-/blob/master/GaudiPolicy/python/GaudiTesting/BaseTest.py#L1328.

fix available with !1489 (merged)