Enable MuonSelectorTools unit test in Athena

This MR makes MuonSelectorToolsTester compatible with running in the Athena environment (in addition to AnalysisBase), and enables the unit tests so that they run in Athena as well. It (finally) follows up !33350 (closed) after the helpful comment from @akraszna there.

I set this to target 21.2 as well, just to keep things in sync, but this is probably not too important in case there is any problem with that.