Bug 1515204 - add mouseup after drop in EventUtils, r=NeilDeakin

Differential Revision: https://phabricator.services.mozilla.com/D28749

--HG--
extra : moz-landing-system : lando

browser/components/customizableui/test/browser.ini

@@ -88,7 +88,6 @@ skip-if = os == "linux" # Intermittent failures
 skip-if = verify
 [browser_956602_remove_special_widget.js]
 [browser_962069_drag_to_overflow_chevron.js]
-skip-if = verify || os == "linux" || os == "mac" && debug # bug 1515204
 [browser_963639_customizing_attribute_non_customizable_toolbar.js]
 [browser_967000_button_charEncoding.js]
 [browser_968565_insert_before_hidden_items.js]

testing/mochitest/tests/SimpleTest/EventUtils.js

@@ -2337,6 +2337,7 @@ function synthesizeDropAfterDragOver(aResult, aDataTransfer, aDestElement, aDest
                                   aDataTransfer, aDragEvent);
     sendDragEvent(event, aDestElement, aDestWindow);
   }
+  synthesizeMouse(aDestElement, 2, 2, { type: "mouseup" }, aDestWindow);
 
   return effect;
 }