Allow bevy to be run as an integration test inside a #[test]

[refnil]

What problem does this solve or what need does it fill?
Make game/project made with bevy as easily testable as possible. It would also help bevy proper since more integration test could be added.

Describe the solution would you like?
I would like to be able to run a bevy app from a #[test] function. In particular, I think it is important to be able to run windowed app.

Describe the alternative(s) you've considered?

• If you only want to check if the code compile, examples could be used. However, that is not really a full testing framework.
• Make a #[bevy::test] attribute that do a bit of the repetitive work of bevy test. Some kind of recommended way of doing thing. I think it is a good idea for the future.

Additional context
I have a working prototype that work for running test on linux with x11. First, let me explain the four problems that I had to solve to make it work.

• Panicking in a bevy app create a nested panic which abort the current process. That prevent the test to continue after the first fail.
• The event_loop used in bevy_init can only be created on the main thread but tests are not run on that thread.
• If you run multiple windowed bevy app simultaneously, window events won't go to the right event loop. That cause a panic when receiving a window events for a window that bevy doesn't know about.
• If you run multiple windowed bevy app sequentially, you can still receive old event that are destined for the previous test.

I intend to make multiple pull request to fix these problems in the coming days. You can check my WIP on https://github.com/refnil/bevy/commits/master.

Two quick questions that would help me make better pull requests?

• Could you force the event loop to drain outside of a bevy app?
• Is it intended/a good practice to panic! in a panic! in the task_pool Drop implementation?

Two inspirations for this issues:

• UnityTest attribute: Create an empty scene to run a test. It could also be a good idea when scene and system loading and unloading is ironed out. Loading a complete bevy app is more general anyway and could fit more purpose.
• Factorio integration testing video: Looks real nice and I want that for bevy haha.

Happy to ear what you think about this.
Thanks for your cool project.

[cart]

Diving into this now. I've started trying to unit test "full" Bevy apps and your changes appear to do the trick. Thanks!

[alice-i-cecile]

This is now resolved for apps that do not require graphics. See this real-life example.

Once #3754 is solved I think this can be closed.

[alice-i-cecile]

Closing now as redundant to #3754 and #2896.