Update Digitization configuration unit test to use SimpleMergeMcEventCollTool (ATLASSIM-6627)

John Derek Chapmanrequested to merge
jchapman/athena:ATLASSIM-6627_23.0 into 23.0

Missed updating this unit test as part of !64049 (merged) Closes ATLASSIM-6627

Tagging @wlampl and @abarton

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