packages: remove tests from base set

Create a new tests package set to hold the tests, which are not useful
to all users and don't belong in the base set.

Reviewed by: emaste, ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52574

(cherry picked from commit 36edc5014f633c1f00031f5fc5a291f8cdd46d83)