Added utilities to alter settings containing lists of values.
Dropped AppCache._empty test utility.

It's now easier to achieve the same effect with override_settings.
Dropped AppCache._without_app test utility.

It's now easier to achieve the same effect with override_list_settings.
Dropped AppCache._with_app test utility.

It's now easier to achieve the same effect with override_list_settings.

I'm finally removing test_missing_app as it is basically impossible to
reach this situation with public methods of the new app cache.