Testing with app settings

A couple of small test fixes that were needed to test modeltranslation alongside framework / project; changes summary:
-- consistently restore active language after each test;
-- reload settings after ``autodiscover`` tests as they may change ``FALLBACK_LANGUAGES`` due to some interplay between ``reload_override_settings`` and the custom installed apps override (maybe someone will be able to trace this one further: fallback languages are sometimes empty on entry to the ``reload_override_settings`` and end up containing the default language on exit);
-- make sure timezone support is disabled for the date / time fields tests;
-- make the file field tests resistant to some other test being first to initialize the default storage with some other media root / location (and hopefully make them work with non-filesystem storages).