Update Fatras-based FastChainPU ART tests

Vincent Pascuzzirequested to merge
vpascuzz/athena:21p3.Fatras_G4MS_ART_tests.update into 21.3

DCube tests were removed previously (!30996 (merged)) due to FATALs in retrieving hard-scatter collections in hit tests (see ATLASSIM-4458).

This update adds back the DCube tests using a postInclude that now looks in the correct StoreGate for the hard-scatter collections (!31179 (merged)).

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