Add PhaseII Pile-up test which saves in-time pile-up truth

Merged John Derek Chapman requested to merge jchapman/athena:FullTruthPUDigiTest_main into master

Adding a basic test of pile-up digitization saving in-time pile-up truth. Relates to discussions in ATLSWUPGR-167

This test will provide a benchmark for this workflow, which can then be used to monitor the clean-up and simplfication of the McEventCollection merging code used during pile-up digitization jobs.

Tagging @tstreble, @tadej, @goetz, @averbyts.

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