Which DB does tinytest use?

If I declare a collection in tinytest on the server, does it actually get written to the disk? My assumption is that minimongo is used for both client and server tests. Is that correct?

No idea on exact details. But, data will be reset on every app restart.

When you run your tests via meteor test-packages, a new temporary meteor app is created that has its own local MongoDB database. You can define an existing MongoDB server by setting the environment variable MONGO_URL.