Remove test warning

[ewjoachim]

There's a warning in the tests:

tests/integration/test_migration.py::test_init_schema_extra_files
  /home/joachim/src/septentrion/septentrion/migration.py:116: UserWarning: ADDITIONAL_SCHEMA_FILES will be deprecated. Use BEFORE_SCHEMA_FILES instead.
    warnings.warn(

-- Docs: https://docs.pytest.org/en/latest/warnings.html

We should catch it and check it, rather than letting it bubble.

Here's the pytest relevant doc: https://docs.pytest.org/en/stable/warnings.html