Clean media files between test runs

[leducvin]

This PR adds cleaning the media files from tests/testapp/test-media between each tox env run. It actually does so before each run (if a user wants to inspect the created files after a test run).

Alternatively, the files could be removed after test runs, leaving it all clean.

Additionnally, it adds tox to the optional testing dependency bundle so that when users initially runs pip install -e .[testing], they are ready to go.

Lastly, it fixes the relative path in the make clean recipe of the makefile.

[zerolab]

Sorry, but this is not an acceptable way. adding rm to allowed externals opens up the door for potential abuse.

The solution should be entirely TestCase-bound. That is, clear the test uploaded media directory on tearDown. e.g.

https://github.com/django/django/blob/f5c944b3141c58bb4a5c7bbca61180b2ad7c13aa/tests/file_storage/tests.py#L733-L735

or

https://github.com/django/django/blob/f5c944b3141c58bb4a5c7bbca61180b2ad7c13aa/tests/staticfiles_tests/test_storage.py#L438-L443

[leducvin]

Oh OK, thanks for the feedback -- I'll change it to do it in tearDown

[leducvin]

Actually, I noticed in

https://github.com/torchbox/wagtailmedia/blob/794bd7e170bb709e5e43084b106b29f34faa9209/tests/manage.py#L57C1-L63C54

that there was already an attempt to clean test media. The problem is that it didn't do anything since the test app used a different MEDIA_ROOT.

So I changed the test app settings to use the same MEDIA_ROOT. Would this be acceptable?