Bug 1490745 - ensure we wait for the actual test page to finish loading, r=mconley

The browserLoaded() promise can be resolved by about:blank loads, it seems.
Being explicit about the load we're waiting for fixes this issue.

Differential Revision: https://phabricator.services.mozilla.com/D50818

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

browser/components/customizableui/test/browser_exit_background_customize_mode.js

@@ -26,8 +26,9 @@ add_task(async function test_exit_background_customize_mode() {
   await BrowserTestUtils.switchTab(gBrowser, nonCustomizingTab);
   await finishedCustomizing;
 
-  BrowserTestUtils.loadURI(custTab.linkedBrowser, "http://example.com");
-  await BrowserTestUtils.browserLoaded(custTab.linkedBrowser);
+  let newURL = "http://example.com/";
+  BrowserTestUtils.loadURI(custTab.linkedBrowser, newURL);
+  await BrowserTestUtils.browserLoaded(custTab.linkedBrowser, false, newURL);
 
   Assert.equal(
     gBrowser.tabContainer.querySelector("tab[customizemode=true]"),