Add an expectation after signing out in officing spec

In this test, we were signing out after confirming a vote, but we
weren't waiting for the request to finish before using the `login_as`
method. Sometimes this test failed with a screenshot showing that the
user wasn't signed in after the `login_as` call, so this might have been
the cause.

We could use the `logout` method instead, but since we don't have any
other tests that click the "Sign out" link from the officing area, we're
leaving it as it was; we might change it to `logout` in the future if
the test keeps failing once in a while.

Note we're also replacing a `visit` call which my guess is was there
because the "Sign out" link wasn't visible due to the notice covering
it. So we're closing the notice instead.

spec/system/polls/polls_spec.rb

@@ -293,8 +293,11 @@ describe "Polls" do
 
       expect(page).to have_content "Vote introduced!"
 
-      visit new_officing_residence_path
+      within("#notice") { click_button "Close" }
       click_link "Sign out"
+
+      expect(page).to have_content "You must sign in or register to continue."
+
       login_as user
       visit poll_path(poll)