Fix flaky user_manages_subscription_spec.rb

It looks like the feature spec was attempting to click on the
subscription button before all the Ajax calls for the page had
loaded. As a result, the click did nothing, and the test failed.

To fix this, we should wait for outstanding requests to be ready before
attempting to click.

Closes https://gitlab.com/gitlab-org/gitlab/issues/207835

spec/features/merge_request/user_manages_subscription_spec.rb

@@ -16,6 +16,8 @@ describe 'User manages subscription', :js do
 
   it 'toggles subscription' do
     page.within('.js-issuable-subscribe-button') do
+      wait_for_requests
+
       expect(page).to have_css 'button:not(.is-checked)'
       find('button:not(.is-checked)').click