Currently all tests must run defer mt.Close after calling mtest.New to make sure mtest cleans up all of the server-side resources it created to run the test. That pattern is error-prone, possibly leading to tests that don't clean up server-side resources.

Instead, we should make the cleanup process automatic by using T.Cleanup and remove mt.Close.

Definition of done:

• Register the mtest cleanup logic as an automatic test cleanup task using T.Cleanup.
• Remove the mtest.T.Close function and all references.