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.

Approved by: re (cperciva)
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

(cherry picked from commit 4b342830abd39cdc20b67a215e15309293673e75)
(cherry picked from commit cdfa9139777189a16ea7a86c6457f169b76d7628)