Run tests at sandbox.translatewiki.net

As soon as this commit is merged into master,
TwnMainPage-sandbox.translatewiki.net-linux-firefox[1] Jenkins job should
not fail with "No test report files were found" error message.

1: https://wmf.ci.cloudbees.com/job/TwnMainPage-sandbox.translatewiki.net-linux-firefox/

Change-Id: Ie419eb2a5ab70561a5da477f1b31f3446b39e552