PhotonVertexSelection: fix unit test
Fix the unit test. Replace the usage of the obscure AthenaStopperAlg
with the builtin feature of AppMgr
. Also make the test actually run on
some events instead of just setting up the job.
cc @akraszna @tadej who have both fiddled with this unit test in previous MRs: !36719 (merged) !39471 (merged)