Bug 1788321, wait for idle time before trying to paint the -ref page, r=dminor

Spellchecker itself is using idle callback for certain things, so this is just trying to mimic that.
But purely guess work. The test is inherently racy since nothing guarantees the OS gives cpu time for the parent side.
We could add some notification that spellchecking is done, but not sure how a reftest coudl then access that notification -
but that would require some changes to the editor/spellchecker side too.

Differential Revision: https://phabricator.services.mozilla.com/D160022

editor/reftests/spellcheck-input-ref.html

@@ -9,14 +9,14 @@
 
       // Try to ensure the input element is repainted.
       let rAFCounter = 0;
-      (function rAF() {
+      requestIdleCallback(function rAF() {
         if (rAFCounter < 10) {
           ++rAFCounter;
           requestAnimationFrame(rAF);
         } else {
           document.documentElement.removeAttribute("class");
         }
-      })();
+      });
     </script>
 </body>
 </html>