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!