Don't overwrite user-media/apps.json with tests

Running tests was overwriting the user-media/apps.json file, which contains the
result of running `./manage.py dump_apps`, which dumps all the apps and
appversions we have, and is used by the amo-validator.

Thus running the tests would overwrite all the apps and appversions previously
updated with `import_prod_versions`, and prevent uploading addons/versions with
"recent" versions.
Doesn't it mean that if you don't have a `apps.json` already these tests will fail ? In fact, doesn't it mean that the while set up here is pointless ? None of the tests in that class will use `MEDIA_ROOT='/tmp'` to load the `apps.json`...