Improve testability of source generators providing Verify into project

### Description

Hi

I tried to make some improvements of the code by making a generator and providing a example how it can looks
but...
I have a problem with that tests because:
- almost all the time you need to exact know what is the output
- updating the output of source generator can be painful
- comparing of "before" and "after" is not easy

Can I provide a pull request with potential way of how improve tests and how we can verify the output?

Also... I'm not sure about license of the external library, if that is acceptable by your team or not
Link to that library: https://github.com/VerifyTests/Verify.SourceGenerators