It is unlikely that you would be doing a test in darkness. The way the test times work you should be doing your test in daylight. That said at certain times of the year and the further north you go the more likely that could be.

What is a greater possibility is that it can become very dark due to cloud or under some kind of cover. The thing here is to be using your lights correctly. If you have your side lights on anyway that is one less thing to think of.

Also make sure you can stop in what you see to be a safe distance.