Tests failing because of left-over files in tests/testapp/test-media

[leducvin]

After cloning the repo, tests fail both on main and on v.0.16.0 because of left-over files in the test media storage. Django is renaming them to prevent conflicts with the existing files.

What I did:

• Cloned the repo, cd-ed into it.
• Created a venv and ran pip install -e '.[testing]' -U
• Ran tox (it was not installed by the previous command, I had to install it)

Typical error that I got after 1st tox env ran:

Traceback (most recent call last):
  File "/home/leducvin/dev/wagtailmedia/tests/test_models.py", line 226, in test_filename
    self.assertEqual("example.mp4", self.media.filename)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'example.mp4' != 'example_ZxhVnHw.mp4'

Don't know if I set up the project wrong...

I can submit a PR for this if you'd like.

[zerolab]

Go for it

[zerolab]

#262 fixes the assertions, but it would still be good to ensure the tests do the appropriate clean-up