Use the real value of MK_TESTS_SUPPORT in _libraries.

We need to build kyua libraries for kyua. Because we set MK_TESTS=no,
we can't not set MK_TESTS_SUPPORT=${MK_TESTS_SUPPORT} because the latter
defaults MK_TESTS_SUPPORT to no.

This fixes WITHOUT_TESTS + WITH_TESTS_SUPPORT builds.

Reported by: Ruslan Garipov
Reviewed by: emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24206