Add regression test for old typechecking issue #505

This adds a regression test for issue #505 (closed), an old type checking bug that resulted in a panic "Non-exhaustive patterns in function readMutTyVar" with 4.11 on a particular RULES pragma. There's an existing test (tcfail093.hs) that references this error message but has to do with functional dependencies, so it's not clear if they're testing the same bug.

Closes #505 (closed)