Bug 1646672 - Fix browser_customize.js on Mac. r=mkmelin

mail/test/browser/shared-modules/CustomizationHelpers.jsm

@@ -56,6 +56,7 @@ CustomizeDialogHelper.prototype = {
     } else {
       ctc = wh.wait_for_existing_window(this._windowType);
     }
+    ctc.sleep(500);
     return ctc;
   },