Now that we generate methods in instantiated generic types, we were
getting test failures from methods that were being detected as new
methods. In actuality, they weren't new, but they differed only in
generic type parameters from some base type method, and
GetSignatureString ignores generic parameters completely.
This fix eliminates the hacky GetSignatureString and replaces it with
more-or-less proper signature comparison. This even manages to fix an
incorrect test case from Methods.cs (because GetSignatureString was
incorrectly incorporating the return type - when the return type should
not be examined for signature checking).
