atf: Guard libexec/atf with WITH_TESTS_SUPPORT

Other atf components are guarded by WITH_TESTS_SUPPORT already.
WITH_TESTS is for actual tests.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47660

(cherry picked from commit 43e045c1733d50fad79f3a53e05140b8dc0d7fa6)