Add tests for asset processing under hot reloading and across asset sources. (#21673)

# Objective

- Trying to rewrite asset processing to be dynamic is hard, especially
since we don't have tests verifying all the situations asset processing
is supposed to be able to handle!

## Solution

- Add a couple more tests! One to check that processing multiple sources
works, and another to check that sending asset events triggers
processing (including processing of dependent processing).

## Testing

- ;)

Author: andriyDev