Fix pytest warnings related to explicit use of test fixture

[drdavella]

This gets rid of several warnings that were occurring when running the tests:

/Users/user/miniconda3/envs/ah-helpers/lib/python3.7/site-packages/_pytest/python.py:198: RemovedInPytest4Warning: Fixture "testpackage" 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.

[astropy-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog and making sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

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