Wait for approval before continuing

After clicking the approve button, this waits for the button to
change to revoke approval. This ensures that in subsequent steps
approval is not still in process (which caused a test to be flaky)

See https://gitlab.com/gitlab-org/gitlab/issues/35525

qa/qa/ee/page/merge_request/show.rb

@@ -79,6 +79,8 @@ module QA
 
           def click_approve
             click_element :approve_button
+
+            find_element :approve_button, text: "Revoke approval"
           end
 
           def start_review