You must not already be streaming during the test, and although TwitchTest needs your stream key, your stream will not go live.
It's measured out of 100, and a quality rating of 80 or higher is recommended for a stable stream.
Streaming at your absolute maximum bitrate will result in lag and possible dropped frames.
Note that TwitchTest will never go above 10mbps during testing and Twitch currently recommend a maximum of 6mbps 6000 kbps when streaming.