We test that the 'hosts' field respects the horizon in repl_ssl_split_horizon.js. After
SERVER-45396, we will test that the 'me' field respects the horizon. We should also test that the 'passives', 'arbiters', and 'primary' fields respect the horizon. (They do, but we do not have test coverage.) These are the remaining fields in the isMaster response that return a string hostname.