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.
`/tmp` would probably break on some envs, maybe using [tempdir](https://docs.python.org/2/library/tempfile.html#tempfile.tempdir) is more appropriated?