Stub system tests for mixed in and inherited methods

This adds some system tests for stubbing methods that are either
mixed in (via include or extend) or inherited. The goal is to
throughly test assert's stubbing in the same way it would be used
in a separate test suite and to find any strange issues that can
come from mixing stubs or using them in strange scenarios. These
include edge cases and error cases that ensure assert's stubbing
tries to alert the users to possible issues as much as possible.
The biggest difference for these kinds of methods is for extended
or inherited class methods assert doesn't check arity when
stubbing or calling the method. For included or inherited instance
methods it does check arity.

This also adds system tests for stubbing a method on a class and
a class that inherits from it. In this case, it stubs `new` on
both the parent class and the child class and ensures they can
be stubbed individually without effecting the other.

@kellyredding - Ready for review.