[Test Framework] Support baseline testing for IL verification and ExprTests

[vzarytovskii]

Baseline testing needs to be supported not only for compilation tests, but also for ILDasm tests & ExprTests.

• Support via the existing "baseline" API in the new compiler
• Support via custom DataAttrbute for xUnit, so it can be used in Theories.

[Happypig375]

Celebration on 10k! 🎉