Tests for Seaworthy itself don't work on pytest 4.0

[JayH5]

Some of our tests for Seaworthy itself don't work on pytest 4 because we call some of the Seaworthy fixtures directly in order to test them. We get errors like:

_pytest.warning_types.RemovedInPytest4Warning: Fixture "docker_helper" called directly. Fixtures are not meant to be called directly, are created automatically when test functions request them as parameters. See https://docs.pytest.org/en/latest/fixture.html for more information.

While Seaworthy itself seems to work fine with pytest 4, this is currently causing builds to fail.

These warnings can be configured to not result in failures, but in pytest 4.1 that will break too.