packages: Remove the tests-dev package

We don't want a tests-dev package, because this means set-devel depends
on tests-dev, which transitively depends on tests, which means you can't
install set-devel without also getting tests.

The only real "dev" files in tests-dev are from ATF (libprivateatf),
so move that to its own package and add a dependency from tests.

Also move Kyua to its own package, since this might be useful for
running tests even when the user doesn't want the whole set of base
tests installed.

Add a dependency from -tests to both -atf and -kyua, and a dependency
on -set-base, since the tests won't work without the full base system
installed.

The remaining "dev" files in tests are actually test artifacts, not real
development libraries. Add a new NO_DEV_PACKAGE option to bsd.lib.mk,
which causes dev files to be installed in the base package instead of
creating a -dev package, and set this option for everything that
installs test libraries.

While here, add a slightly more informative description for the tests
package.

MFC after: 3 seconds
Reviewed by: bapt, emaste
Differential Revision: https://reviews.freebsd.org/D52597

(cherry picked from commit 9065390ddc7b1ce2613777f6a947200950a5ac71)