Improve type validation tests

[fornellas]

Add more thorough testing to type checking:

• tests/lib_testslide.py added with more cases variation.
• StrictMock & mock_callable() had type checking tests added for all use cases (module function / instance/class/static methods).
  • Found an issue with type checking for class methods.

Along the way:

• Fixed type checking for class methods.
• Fixed type checking error message.
  • It was an aggregation of TypeError messages which would not match the existing error.
  • It now shows each error on a separate line:

TypeError: Call with incompatible argument types:
  'arg1': type of arg1 must be str; got int instead
  'arg2': type of arg2 must be str; got int instead
  'kwarg1': type of kwarg1 must be str; got int instead
  'kwarg2': type of kwarg2 must be str; got int instead"

• Moved all common stuff to testslide/lib .
• Integrated common logic for extracting methods from mocks into _wrap_signature_and_type_validation, so StrictMock & mock_callable now works with other mocks.