Fixes #20502 -- i18n test bug discovered by Travis.

If the following tests are run in this order:

* defaultfilters.tests.DefaultFiltersTests.test_localized_filesizeformat
* i18n.tests.AppResolutionOrderI18NTests

then the second test will fail because the first has cached the 'de' translation before the second one changes INSTALLED_APPS to install the test app, which has its own translation.