Google operates http://connectivitycheck.gstatic.com/generate_204 and http://www.gstatic.com/generate_204, both of which return a 204 No Content response.

Apple operates http://captive.apple.com, which returns `<HTML><HEAD><TITLE>Success</TITLE></HEAD><BODY>Success</BODY></HTML>`.

Microsoft operates http://www.msftconnecttest.com/connecttest.txt, which returns `Microsoft Connect Test`.

Mozilla operates http://detectportal.firefox.com, which returns `success`.

These are what these companies' respective browsers/operating systems use to check if they have an internet connection and are not stuck behind a captive portal. Given the enormous quantity of devices out in the world that are relying on these URLs to stay up, you can probably feel comfortable using these in your tests. If they do go down, good chance you'll see news articles about it before you notice your tests failing.