If I wanted to write tests for a migration in joplin, is there a mechanism to help me?

In the Python ORM we’re using (Axiom), it has some test harness stuff that allows you to write a “stub” that you run (at the current database version) that creates a self-contained database, then you write historic tests against that database to ensure your migrator does the things it is supposed to.

This gives one some ability to test all the edge cases a migrator might have to deal with in a controlled environment.