Bug 1812493 - Make the test wait for "load" of iframe if it's not yet ended r=m_kato

Differential Revision: https://phabricator.services.mozilla.com/D170029

editor/libeditor/tests/test_nsIEditorMailSupport_insertTextWithQuotations.html

@@ -12,8 +12,15 @@
 "use strict";
 
 SimpleTest.waitForExplicitFinish();
-SimpleTest.waitForFocus(() => {
+SimpleTest.waitForFocus(async () => {
   const iframe = document.querySelector("iframe");
+  await new Promise(resolve => {
+    if (iframe.contentDocument?.readyState == "complete") {
+      resolve();
+      return;
+    }
+    iframe.addEventListener("load", resolve, {once: true});
+  });
 
   function testInDiv() {
     const inPlaintextMode = getEditor(window).flags & SpecialPowers.Ci.nsIEditor.eEditorPlaintextMask;