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.
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
packages.
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