tests/artifacts/PURPOSE

tests: add a `tests/artifacts/` directory

That directory is meant to cache large items used by tests that are slow to
generate. See 'PURPOSE' file for details and next changesets for a first user.

This directory is meant to cache artifacts useful for tests (such as bundle).

Those artifacts need to be cached because they are slow to regenerate on each
test but too large to be tracked within the repository. They are not expected
to change between each run and can be cached.

The `./scripts/` contains code to generate the artifact while the `cache`
directory contains resulting artifact.