Refactor unit tests

src/cutecoin/gui/mainwindow.py

src/cutecoin/tests/__init__.py

src/cutecoin/tests/all.py

src/cutecoin/tests/main_window/__init__.py

src/cutecoin/tests/main_window/test_main_window_dialogs.py

src/cutecoin/tests/main_window/test_main_window_menus.py