tests: Fix failing tests

[ParthS007]

• https://github.com/inveniosoftware/flask-menu/runs/1524597992?check_suite_focus=true

FAILED tests/test_core.py::TestMenu::test_active_checks_segment_not_prefix - ...
FAILED tests/test_core.py::TestMenu::test_active_item - AttributeError: 'Asse...
FAILED tests/test_core.py::TestMenu::test_active_when - AttributeError: 'Asse...
FAILED tests/test_core.py::TestMenu::test_app_without_existing_extensions - A...
FAILED tests/test_core.py::TestMenu::test_blueprint - AttributeError: 'Assert...
FAILED tests/test_core.py::TestMenu::test_classy_endpoint - AttributeError: '...
FAILED tests/test_core.py::TestMenu::test_classy_endpoint_on_blueprint - Attr...
FAILED tests/test_core.py::TestMenu::test_classy_endpoint_with_args - Attribu...
FAILED tests/test_core.py::TestMenu::test_double_instantiation - AttributeErr...
FAILED tests/test_core.py::TestMenu::test_dynamic_blueprint_with_auto_args - ...
FAILED tests/test_core.py::TestMenu::test_dynamic_list_constructor - Attribut...
FAILED tests/test_core.py::TestMenu::test_dynamic_url - AttributeError: 'Asse...
FAILED tests/test_core.py::TestMenu::test_dynamic_url_with_auto_args - Attrib...
FAILED tests/test_core.py::TestMenu::test_external_url - AttributeError: 'Ass...
FAILED tests/test_core.py::TestMenu::test_has_visible_child - AttributeError:...
FAILED tests/test_core.py::TestMenu::test_kwargs - AttributeError: 'Assertion...
FAILED tests/test_core.py::TestMenu::test_kwargs_override - AttributeError: '...
FAILED tests/test_core.py::TestMenu::test_simple_app - AttributeError: 'Asser...
FAILED tests/test_core.py::TestMenu::test_visible_when - AttributeError: 'Ass...
FAILED tests/test_core.py::TestMenu::test_visible_when_with_dynamic - Attribu...

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