Bug 1723511: Update tests within dom/xhr to work with https-first enabled r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D121528

dom/xhr/tests/browser_sync_xhr_event_handing_switch_bcg.js

@@ -1,6 +1,6 @@
 const baseURL = getRootDirectory(gTestPath).replace(
   "chrome://mochitests/content",
-  "http://example.com"
+  "https://example.com"
 );
 
 const childURL = `${baseURL}empty.html`;