Add tests for asset processing to show the problem with #21269. (#21433)

# Objective

- Make the problem with #21269 more concrete.

## Solution

- Add a test to show that loaders during processing can read processed
assets.
- Add an ignored test to show that loaders during processing cannot
currently read source assets.
- This also tests that nested asset loads during processing actually
works.

## Testing

- Oops! All tests!

Author: andriyDev