Permit underscores in method names without disabling INTL0003 entirely

> DO NOT: Use underscores to break up words in an identifier. **Test methods**, UI generated event handlers and references to Resource files **are the exception**.

For test methods, I need to suppress `INTL0003` as it checks for both PascalCasing and a lack of underscores. If the underscores check was extracted into a separate analyzer, I could suppress underscores without killing the PascalCasing check wholesale.