Add test for SSO Push updates to emails_controller_test

As far as I can tell we've been sending push updates (to the applications
a user has access to) when a user's email changes since the functionality
was added to Signon[1] and gds-sso[2] in 2012.

Although we're still sending those push updates from the new
`Users::EmailsController` (added in #2509) we weren't testing it in the
controller test. I'm adding a test (copied from the previous commit) to
avoid regressions in future.

[1]: f339848
[2]: alphagov/gds-sso@8c0888c

test/controllers/users/emails_controller_test.rb

@@ -166,6 +166,13 @@ class Users::EmailsControllerTest < ActionController::TestCase
         put :update, params: { user_id: user, user: { email: "[email protected]" } }
       end
 
+      should "push changes out to apps" do
+        user = create(:user)
+        PermissionUpdater.expects(:perform_on).with(user).once
+
+        put :update, params: { user_id: user, user: { email: "[email protected]" } }
+      end
+
       should "redirect to edit user page and display success notice" do
         user = create(:user, email: "[email protected]")