Can tested yeast be used for anything?

You can certainly use it immediately in the place of yeast in a recipe - you basically have done a pre-activation of the yeast. This is a common part of many recipes.

You should also be able to store it in the fridge for at least a few days and then add it to a bread recipe without any problems. Yeast are quite hardy and will survive fairly well over that time.

You could also dry it down in parts onto sheets of foil, wrap them up and use them to inoculate a new mixture, you can then expand this mixture to propagate them indefinitely - free yeast!

Yeast can most definitely be used if it's alive. Just be aware that you will need to keep feeding it or it will die. If you put it into the fridge that will slow down its metabolism. Yeast is all around us and you can make starter easily with no yeast simply by adding water to flour and waiting (yeast is already in the flour and in the air and crawling up your backside). The main thing to keep in mind is that if you're making something like bread that is pretty forgiving of amount of yeast used, you really just have to monitor the progress of the prove(s), so that's a great way to use your yeast. If you're making a recipe that's more exacting of amount used and is perhaps baked straightaway without proofing then you may want to add the amount stated in the recipe. Also, in your testing method you added sugar. Some of that sugar will still be in your solution and some will have been metabolized and converted to alcohol. Depending on your recipe and the amount you're making the added sugar could affect browning, texture, etc.