Some of this is kind of subjective, but you want to be able to give testers something do to. You don't want them resigning immediately, and you don't want to leave them stuck.
- First, as a precaution, either tell the testers via email, or block them in-game, if an area isn't available yet. This isn't testing per se, but it does make sure their time isn't wasted.
- If you add a new feature, you should give it basic testing.
- If your programming language allows automated walkthroughs, you should try that or those, especially with alternative ways through. For instance, test maze = "sw/ne/w/n/e/nw/u/s"
- The introduction is important, not just because you want to make a good first impression, but also because testers don't like getting stuck right away.
A useful trick is to replay the game after being away from it for a few hours. Try to walk through yourself. Note anything that is odd.
It's also useful to have a checklist for basic stuff, so you aren't wondering if you tested something or not. That takes a lot of energy to remember.