atf: Move the tests back to the tests package

Commit 9065390ddc7b moved atf to its own package, but mistakenly moved
the tests as well. Put the tests back into the test package.

Fixes: 9065390ddc7b ("packages: Remove the tests-dev package")
MFC after: 1 day
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53594