Tests: Allow statusText to be "success" in AJAX tests

Tests: Allow statusText to be "success" in AJAX tests

In HTTP/2, status message is not supported and whatever is reported as
statusText differs between browsers. In Chrome & Safari it's "success", in
Firefox & IE it's "OK". So far "success" wasn't allowed. This made the tests
pass locally if you're running an HTTP/1.1 server but on TestSwarm which is
now proxied via an HTTP/2-equipped Cloudflare, the relevant test started failing
in Chrome & Safari.

Allow "success" to resolve the issue.

Closes gh-4973