Bug 1721217 - Part 11: Fix a race between two parent-initiated loads, r=nika

Wait for the previous load to complete as sometimes the first load takes longer
to start and races with the latter about:mozilla one. A follow up bug has been
filed to address this issue - Bug 1742886.

Differential Revision: https://phabricator.services.mozilla.com/D131263

dom/security/test/general/browser_test_referrer_loadInOtherProcess.js

@@ -142,6 +142,8 @@ add_task(async function test_navigation() {
 
   // Navigate from remote to non-remote
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TEST_PAGE);
+  // Wait for the non-blank page to finish loading
+  await BrowserTestUtils.browserStopped(gBrowser, TEST_PAGE);
   testURI = "about:mozilla";
   permanentKey = gBrowser.selectedBrowser.permanentKey;
   await waitForLoad(testURI);