feat(watch): add on-change cli option

[dolsem]

#1602 and #1366 described the use case for executing a piece of code before all tests. I have a similar use case where I need to wipe my test database each time the tests are run. This is more of a problem when AVA is in watch mode because I need to run my logic every time it detects a change. So I propose a CLI option to run a custom shell command every time the test are re-run.

[novemberborn]

We've intentionally not added this for regular test runs. I don't think we should add it for watch mode either.

Something like #1366 would be a better fit — as in: you can load code in AVA's main process and it gets some lifecycle events allowing it to do stuff before tests run. #2478 lays the groundwork for that. It's pretty close to shipping experimentally actually. Maybe you could open an issue to explore that further?

Sorry to disappoint. I do understand where you're coming from, and thank you for figuring out how to make it work and opening a pull request. It's just a question of where to fit it into AVA.

FWIW, I write a lot of service integration tests which rely on Firestore. For that database the emulator has a neat trick: if you connect using a new project ID you get a clean database. So the execution of each test file is isolated from the others.

[dolsem]

@novemberborn
Got it. #1366 seems like a much more complex way to solve this problem. In any case, is there anything I can do to help get #2478 done and merged? How soon do you think it will be finished?

[novemberborn]

#2478 is close to shipping experimentally, and then we can figure out how to make it work with the run lifecycle.

I appreciate that all of this seems like more effort. Ultimately the goal is to enable many interesting use cases with code that can even be shared through the package registry.