Consider migrating to Munit

[LukaJCB]

ScalaTest is making it quite hard to keep up with the dotty milestones, we really don't need any of the functionality that ScalaTest provides, so migrating to Munit should be fairly straightforward, even if it's a lot of work.

[travisbrown]

I very strongly support this move. I've had only good experiences with MUnit (in DhallJ, an in-progress Circe migration, etc.).