Bug 1511465: Fix race condition in browser_oversized.js. r=mak

Seems that when run as the first test in a test run there is a race for whether
the favicon for the initial tab has already been seen or not. Rarely we fail the
race and end up seeing a successful favicon load. This makes us ignore any
favicons other than the one we're interested in.

Differential Revision: https://phabricator.services.mozilla.com/D13838

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

browser/base/content/test/favicons/browser_oversized.js

@@ -5,7 +5,7 @@ const ROOT = "http://mochi.test:8888/browser/browser/base/content/test/favicons/
 
 add_task(async () => {
   await BrowserTestUtils.withNewTab({ gBrowser, url: "about:blank" }, async (browser) => {
-    let faviconPromise = waitForFaviconMessage();
+    let faviconPromise = waitForFaviconMessage(true, `${ROOT}large.png`);
 
     BrowserTestUtils.loadURI(browser, ROOT + "large_favicon.html");
     await BrowserTestUtils.browserLoaded(browser);