Bug 1588142 - Test fixes r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D51714

--HG--
extra : moz-landing-system : lando

toolkit/content/tests/browser/browser_label_textlink.js

@@ -9,7 +9,7 @@ add_task(async function() {
         label.href = newTabURL;
         label.id = "textlink-test";
         label.textContent = "click me";
-        doc.documentElement.append(label);
+        doc.body.append(label);
       });
 
       // Test that click will open tab in foreground.