* Addressing #4071 Wrote an ImmutableReturnMixin to prevent developers overriding return_value in certain Mock objects
* Language
* Loosening the assumption that underlying _mock objects need to be Immutable-like simplifies implementation
* Addressing #4071
* Ensure side_effects and return_values are pushed down to the underlying _mock in FreezableMocks. And IDisplay mocks are no longer frozen in _create_get_utility_mock()
* Edit a handful of tests to not override the mock_get_utility return_value
* Brief explainer of FreezableMock.__setattr__
* Incorporating PR feedback and some compatibility
* FreezableMock __getattr__ needs a shortcut in case of return_value or side_effect
* Changing return_value only forbidden if set before freezing
* Remove unnecessary else block
* Expanded doc strings
* Bring a couple new tests in line with patch_get_utility() norms
* Addressing #4434 by implementing ConfigTestCase which mocks out a NamespaceConfig for consistent config use across tests
* Refactor account_test.py for use with ConfigTestCase
* Remove superfluous setup/teardown
* Pylint oops.
* Fix redundant inheritance class definitions
* Separate ConfigTestCase's mocked directories
* Module import style consistency
* Refactor log_test.py for use with ConfigTestCase
* Refactor eff_test.py for use with ConfigTestCase. Also tweak for import style consistency
* Refactor reverter_test.py for use with ConfigTestCase
* Refactor renewal_test.py for use with ConfigTestCase
* Refactor main_test.py for use with ConfigTestCase
* Refactor storage_test.py for use with ConfigTestCase
* Refactor cert_manager_test.py for use with ConfigTestCase
* Refactor client_test.py for use with ConfigTestCase
* Refactor configuration_test.py for use with ConfigTestCase
* Pylint!
* Incorporating PR feedback
* Remove comment
