Bug 1455078: Enable service worker testing in test r=asuth

This preference must be enabled since the test creates an insecure context, in
which navigator.serviceWorker is undefined.

Differential Revision: https://phabricator.services.mozilla.com/D3594

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

dom/serviceworkers/test/test_navigator.html

@@ -28,7 +28,8 @@
 
   SpecialPowers.pushPrefEnv({"set": [
     ["dom.serviceWorkers.exemptFromPerDomainMax", true],
-    ["dom.serviceWorkers.enabled", true]
+    ["dom.serviceWorkers.enabled", true],
+    ["dom.serviceWorkers.testing.enabled", true]
   ]}, function() {
     checkEnabled();
     SimpleTest.finish();