Bug 1607984 - P6. Fix test. r=MattN

Following the changes to DocumentChannel the test was failing.
With DC, a load may take a few event loops to start. This current test was only waiting for the load to start to the URL about:preferences#privacy-logins and would immediately tear down the window.
However, this URL redirects to about:preferences#privacy ; destroying the window midway could cause XML parsing error.

So now we wait for the page to fully load, and make sure we've been through both addresses.

Differential Revision: https://phabricator.services.mozilla.com/D70003

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

browser/components/aboutlogins/tests/browser/browser_openPreferences.js

@@ -12,9 +12,28 @@ add_task(async function setup() {
 });
 
 add_task(async function test_open_preferences() {
+  // We want to make sure we visit about:preferences#privacy-logins , as that is
+  // what causes us to scroll to and highlight the "logins" section. However,
+  // about:preferences will redirect the URL, so the eventual load event will happen
+  // on about:preferences#privacy . The `wantLoad` parameter we pass to
+  // `waitForNewTab` needs to take this into account:
+  let seenFirstURL = false;
   let promiseNewTab = BrowserTestUtils.waitForNewTab(
     gBrowser,
-    "about:preferences#privacy-logins"
+    url => {
+      if (url == "about:preferences#privacy-logins") {
+        seenFirstURL = true;
+        return true;
+      } else if (url == "about:preferences#privacy") {
+        ok(
+          seenFirstURL,
+          "Must have seen an onLocationChange notification for the privacy-logins hash"
+        );
+        return true;
+      }
+      return false;
+    },
+    true
   );
 
   let browser = gBrowser.selectedBrowser;