Fix missing expectation in budget polls spec

We've had a failure in one of our test suite runs [1]. A possible cause
could be an HTTP request taking place at the same time as an AJAX
request, with both trying to access the database at the same time.

We've had several similar issues in the past which have been fixed by
checking the AJAX request has finished before requesting another page,
so we're applying the same principle here.

[1] https://github.com/consul/consul/runs/1855207922

spec/system/budget_polls/voter_spec.rb

@@ -54,6 +54,8 @@ describe "BudgetPolls", :with_frozen_time do
 
       within("#poll_#{poll.id}") do
         click_button("Confirm vote")
+
+        expect(page).to have_content "Vote introduced"
       end
 
       visit new_officing_residence_path