Bug 1716516: Fix race in test_allowJavascript.

Differential Revision: https://phabricator.services.mozilla.com/D117951

docshell/test/unit/test_allowJavascript.js

@@ -239,8 +239,10 @@ add_task(async function() {
 
     oopFrame1.reload(0);
     inprocFrame1.reload(0);
-    await AllowJavascriptParent.promiseLoad(oopFrame1);
-    await AllowJavascriptParent.promiseLoad(inprocFrame1);
+    await Promise.all([
+      AllowJavascriptParent.promiseLoad(oopFrame1),
+      AllowJavascriptParent.promiseLoad(inprocFrame1),
+    ]);
 
     equal(
       oopFrame1.currentWindowGlobal.allowJavascript,