Fix output file comparisons in test_Digi_tf_mc20a_qballs.sh ART job (ATLASSIM-6570)

John Derek Chapmanrequested to merge
jchapman/athena:FixATLASSIM-6570_23.0 into 23.0

Fix for OLDvsFixedRef and regression checks in test_Digi_tf_mc20a_qballs.sh ART job. Should fix ATLASSIM-6570.

NB This merge request only affects an ART script, so will have no effect on the CI output.

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