Use `afterTree` to describe dependencies in `clash-testsuite`

[martijnbastiaan]

See https://github.com/martijnbastiaan/tasty/pull/2. This reduces the startup time of clash-testsuite from ~half a minute to sub second figures.

I still have some i's to dot and t's to cross for that PR, so let's wait a bit to see what the maintainers of Tasty think when I open a PR on the main repo.

Still TODO:

• [x] ~~Write a changelog entry (see changelog/README.md)~~
• [x] ~~Check copyright notices are up to date in edited files~~