change `Repo.get(Activity, id)` => `Activity.get_by_id(id)` in tests

test/user_test.exs

test/web/activity_pub/activity_pub_test.exs

test/web/activity_pub/transmogrifier_test.exs

test/web/mastodon_api/mastodon_api_controller_test.exs

test/web/mastodon_api/notification_view_test.exs

test/web/ostatus/activity_representer_test.exs

test/web/ostatus/incoming_documents/delete_handling_test.exs

test/web/ostatus/ostatus_test.exs

test/web/twitter_api/twitter_api_controller_test.exs

test/web/twitter_api/views/activity_view_test.exs