Add easier way to switch to reflection mode

[AdmiralSnyder]

I'd like to get your opinion on making it easier to switch a test project to reflection mode by tunit respecting the assembly attribute TUnitReflectionScanner (or some other attribute)

this is required for getting source generated unit tests (like the ones created with bUnit) integrated properly (in VS Test explorer, at least)

details on the problem are found here:
#3026 (comment)
there's a workaround configuring a .runsettings file which does work but messes with environment variables indirectly, so i don't like that.
i'd like to have your opinion first before i PR something. it would basically mean adding another condition in GetUseSourceGeneration()