Register pytest mongo marker to avoid warning.

See https://docs.pytest.org/en/stable/mark.html#registering-marks

common/lib/pytest.ini

@@ -8,6 +8,8 @@ filterwarnings =
     default
     ignore:No request passed to the backend, unable to rate-limit:UserWarning
     ignore::xblock.exceptions.FieldDataDeprecationWarning
+markers =
+    mongo
 norecursedirs = .cache
 python_classes =
 python_files = tests.py test_*.py tests_*.py *_tests.py __init__.py