Add test coverage for ResX classes

[JeremyKuhne]

We recently ported StronglyTypedResourceBuilder and that has some end-to-end tests, but we don't have good coverage for ResxResourceReader and ResxResourceWriter on their own.

BinaryFormatter depreciation makes filling in test coverage here much more important.

This issue is now marked as "help wanted", and we’re looking for a community volunteer to work on this issue. If we receive no interest in 180 days, we will close the issue. To learn more about how we handle feature requests, please see our documentation.

Happy Coding!

[ssflynn77]

I created a PR that adds a few additional tests for the ResxResourceReader - #8955. Let me know if this looks like what you are looking for, and I will create additional tests for the Reader and the Writer.