Interface/InteropTest case assertion refactoring

This will allow multiple sub-assertions to be performed---see next commit.
Interface.isInstanceOf will account for interop compatibility

This is a bug fix.

If the provided object's constructor is an ease.js type, then the
conventional rules will apply (as mentioned in the test docblock and in the
manual); however, if it's just a vanilla ECMAScript object, then the interop
compatibility checks will be used instead.

The manual already states that this is the case; unfortunately, it
lies---this was apparently overlooked, and is a bug.
Interface.isInstanceOf now accounts for interface interop

More information in the merged commit message.