Refactor: improve mocking in tests

[mxsasha]

IRRD's tests use a lot of mocking, but it's all a bit messy, especially when mocking database results. We may be able to extract a common mock configurator that will improve clarity of tests. Especially for the commonly mocked parts, like database and RPKI validator. Some places also still use the legacy method of overriding settings with env variables, rather than the new config_override() fixture.