Add tests for macro output

[teor2345]

Currently it is hard to modify the macro code, because we don't have any tests that compare the new macro output with the old macro output. This means any (unintended) differences are harder to see.

We should have tests for every major code path:

• Free functions
• Methods
• Associated functions (class functions)

Any significant variations within this code should also have output tests, for example, when we support:

• Lifetimes
• Generics
• Different self types