Add correct exit value to failed tests

Sentinel tests are currently failing, but it took me a while to notice because the return value from sentinel (and cluster) testing wasn't triggering an error.  I applied this commit to my travis-ci branch and now the tests show as failed.

See https://travis-ci.org/mattsta/redis (also, ci.redis.io has stopped updating again and nobody seems to notice.  Adding automatic travis CI testing sure would catch a lot of problems without resorting to manual testing :octocat:)