DigitizationTests ART: Fixes for regression tests

Some output comparisons to previous nightlies were failing due to typos in the art.py compare grid syntax, mostly due to missing variable definitions. These should all be fixed now.

Simulation/Tests/DigitizationTests/test/test_Digi_tf_RUN2_2018_ttbar.sh will take one additional nightly for the regression test to pass as there the issue was that the output file was not being saved, rather than an issue with the regression test command-line.

Also added Simulation/Tests/DigitizationTests/test/test_Digi_tf_RUN2_2015_heavy_ion_hijing.sh which will replace Simulation/Tests/DigitizationTests/test/test_Digi_tf_mc15_2015_heavy_ion.sh.

Part of the work for ATLASSIM-7059