Workaround for epic spec failure

For some reason when a certain set of specs are run together, the input to the
epic field gets stuck waiting for more text. Clicking the "Add" button only
causes it to get out of this state, but it doesn't actually submit the
request. By sending a keystroke before clicking the button, we appear to get
more consistent behavior.

Closes #4185

ee/spec/features/epics/epic_issues_spec.rb

@@ -55,6 +55,10 @@ describe 'Epic Issues', :js do
     def add_issues(references)
       find('.related-issues-block h3.panel-title button').click
       find('.js-add-issuable-form-input').set(references)
+      # When adding long references, for some reason the input gets stuck
+      # waiting for more text. Send a keystroke before clicking the button to
+      # get out of this mode.
+      find('.js-add-issuable-form-input').send_keys(:tab)
       find('.js-add-issuable-form-add-button').click
 
       wait_for_requests