Fix the skipped tests (bug 1050754)

Fixes [bug 1050754](https://bugzilla.mozilla.org/show_bug.cgi?id=1050754)

Some notes:
- `amo.tests.test_decorators:test_write`: this test isn't relevant to what's being tested. The fix is to use [a post request task](https://github.com/mozilla/zamboni/blob/master/lib/post_request_task/task.py)
- `browse.tests`: the `Search Tools` page doesn't have a "featured" filter anymore, see [bug 1084045](https://bugzilla.mozilla.org/show_bug.cgi?id=1084045)
- `lib/es/tests/test_commands.py` removed: it was testing that it was still possible to index some addons while a reindex was occuring. This was a good test to have, but:
  - it's always been a pain to have working correctly (fiddling with threads and stuff, aouch)
  - we're not using aliases in tests anymore, since the last refactor for the ES upgrade (and reintroducing them would be a pain)
- `addons.tests.test_views:TestMobileHome.test_addons`: there might be a corner case where there's less than 3 featured addons: there could be one of the 3 random featured IDs that correspond to a Persona, in that case it won't be displayed here (this happens in the mobilized version of `addons.views.home`)

Remaining skipped test: `translations.tests.test_models:TranslationMultiDbTests.test_translations_reading_from_multiple_db_using`. This test needs a patch on `django-queryset-transform` to obey the `using` properly. Can't be fixed without it, but we still hope to have something fixed on `django-queryset-transform` one day.
Might deserve a comment (same as in PR desc?) to explain the reason we can't test that here for future attempts?