tests: move remove_all_notifications outside of BaseTestCase In preparation of allowing real pytest-style test cases (instead of unittest-style ones), some reorganization is needed in the base test classes, for one because we want a transition period where pytest and unittest style test cases can live alongside each other, and secondly because the pytest style test classes cannot have an __init__ method. The BaseTestCase class will not be reused for the pytest test cases, but the remove_all_notifications method will. To avoid having to duplicate it, and since it does not use any resources from the class (self), move the method out of the BaseTestCase class to top-level, and export it in kallithea.tests.