Fix test suite name in src.conf(5).

There is no such thing as the "Kyua test suite"; it is the "FreeBSD Test
Suite".

While doing this, point readers to tests(7).
Regen after rewording of WITH_TESTS.
Document how to install the test suite.

As part of this, install the tests(7) manual page unconditionally (not only
when WITH_TESTS=yes) so that users that have not yet enabled the build of
the test suite can read details on how to do so.