Bug 1599618 - Fix browser_stop_pending.js in Fission mode. r=adw

The origin switch causes a process switch that makes us load about:blank first.
For the scope of the test we can just check we don't revert to the original url
and we finally set the expected one.

Differential Revision: https://phabricator.services.mozilla.com/D55524

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

browser/components/urlbar/tests/browser/browser.ini

@@ -96,7 +96,6 @@ support-files =
   searchSuggestionEngineSlow.xml
   searchSuggestionEngine.sjs
 [browser_stop_pending.js]
-fail-if = fission
 support-files =
   slow-page.sjs
 [browser_switchTab_closesUrlbarPopup.js]

browser/components/urlbar/tests/browser/browser_stop_pending.js

@@ -30,15 +30,23 @@ add_task(async function() {
     true
   );
 
+  let initialValue = gURLBar.untrimmedValue;
   let expectedURLBarChange = SLOW_PAGE;
   let sawChange = false;
-  let handler = e => {
-    sawChange = true;
-    is(
+  let handler = () => {
+    isnot(
       gURLBar.untrimmedValue,
-      expectedURLBarChange,
-      "Should not change URL bar value!"
+      initialValue,
+      "Should not revert URL bar value!"
     );
+    if (gURLBar.getAttribute("pageproxystate") == "valid") {
+      sawChange = true;
+      is(
+        gURLBar.untrimmedValue,
+        expectedURLBarChange,
+        "Should set expected URL bar value!"
+      );
+    }
   };
 
   let obs = new MutationObserver(handler);