Making Simulation AthenaMT ART output more useful.

Adding --error-mode resilient to all diff-root comparisons of Athena vs AthenaMT output in the DigitizationTestsMT, ISF_ValidationMT, OverlayTestsMT and SimCoreTestsMT ART packages.

This should mean that all leaves with differences are reported rather than all leaves until the number of leaves differs between the two files.