I think the test will always knock out the DSL connection.
It's a long time since I last used it, but I just left the page sitting there, the DSL re-connected, and eventually the test result came up. It may depend on your router, how quickly it re-connects, and how quickly things time out due to your web browser, firewall and/or operating system. A router that re-directs you to some sort of page to tell you your connection is down is probably more likely to mess things up.