Resolve a transient failure in MWPS feature spec

When this spec is run in a certain order, there appeared to be a timing
issue with opening the "Merge when pipeline succeeds" drop-down menu. We
now explicitly wait for any pending requests to complete before clicking
the drop-down.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6770

spec/features/merge_request/user_merges_when_pipeline_succeeds_spec.rb

@@ -88,6 +88,8 @@ describe 'Merge request > User merges when pipeline succeeds', :js do
 
     describe 'enabling Merge when pipeline succeeds via dropdown' do
       it 'activates the Merge when pipeline succeeds feature' do
+        wait_for_requests
+
         find('.js-merge-moment').click
         click_link 'Merge when pipeline succeeds'