Fix ES tests on amo (bug 858447)

This PR, together with PR #1614 and #1594 should fix all ES failing tests for AMO.

Details:
* adding a "Addon.save" in the addon_factory fixed apps.browse.tests.TestUpdatedSort and apps.search.tests.test_view.TestPersonaSearch
* mocking index_webapps fixes tests apps.search.tests.test_views.TestSearchSuggestions
* PR #1614 avoids slug clashes in apps.search.tests.test_view.TestPersonaSearch
* PR #1594 avoids apps.search.tests.test_views.TestESSearch issues with RequestFactory having no GET attribute