Bug 1797858 - Don't leave test without content task promise resolved. r=Jamie

Differential Revision: https://phabricator.services.mozilla.com/D160575

accessible/tests/browser/e10s/browser_caching_domnodeid.js

@@ -16,7 +16,7 @@ addAccessibleTask(
     // We don't await for content task to return because
     // we want to exercise the untilCacheIs function and
     // demonstrate that it can await for a passing `is` test.
-    invokeContentTask(browser, [], () => {
+    let contentPromise = invokeContentTask(browser, [], () => {
       content.document.getElementById("div").id = "foo";
     });
 
@@ -25,6 +25,9 @@ addAccessibleTask(
       "foo",
       "ID is correct and updated in cache"
     );
+
+    // Don't leave test without the content task promise resolved.
+    await contentPromise;
   },
   { iframe: true, remoteIframe: true }
 );