Override MEDIA_ROOT as well as MEDIA_URL.

This has two desirable effects:

1) The files created by the tests are in a temp directory which is
deleted at the end, so don't end up cluttering up the real MEDIA_ROOT
directory.

This could be done better with a context manager - probably in a class
decorator, but the temporary directory should be deleted unless things
go wrong.

2) FileSystemStorage keeps its own copy of MEDIA_URL, so
override_settings(MEDIA_URL=...) doesn't make all the tests pass all
the time - exactly which ones fail and when seems to vary depending on
the system.