Follow up to add additional tests to Logging generator

[dariusclay]

@dariusclay / @RussKie - I've backed out the changed to the public API and added some more tests. CI is now green. Let me know once you've reviewed.

I won't get to it, but I also recommend (either as a prereq for this to merge) or as a followup:

• Adding a test directly against the Type of each value in state.TagArray. We're incidentally testing it with my test additions, but a more targeted test would be ideal.
• Adding tests directly for TypeSymbolExtensions.{ImplementsIConvertible,ImplementsIFormattable,ImplementsISpanFormattable and the newly introduced helper, TypeSymbolExtensions.GetPossiblyNullWrappedType. This is incidentally being tested with my test additions, but not directly.

Thanks!

Originally posted by @rwoll in #5416 (comment)