Update DigitizationTests pile-up event info

Vincent Pascuzzirequested to merge
vpascuzz/athena:master.ATLASSIM-4467.fix into master

Pile-up is now managed by xAOD::EventInfo. PileUpEventInfo is therefore no longer used, so PileUpEventInfoTest and associated tool are renamed to reflect the changes: PileUpEventInfoTest{Tool} -> EventInfoTest{Tool}

Also took this opportunity to adopt use of new-style StoreGate retrieves.

Closes ATLASSIM-4467

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