Bug 1625500 - fix use of .then(x, x) in the tree, r=marionette-reviewers,Standard8,whimboo

Differential Revision: https://phabricator.services.mozilla.com/D68614

toolkit/components/url-classifier/tests/mochitest/test_donottrack.html

@@ -127,7 +127,7 @@ let loop = function loop(index) {
     loop(index + 1);
   };
   let result = executeTest(test);
-  result.then(next, next);
+  result.finally(next);
 };
 
 SimpleTest.waitForExplicitFinish();