testing: move atf-pytest-wrapper to /usr/libexec

Move pytest wrapper to the collection of the other atf wrappers
in libexec. It solves the problem of combining bits & pieces from
bsd.test.mk and bgs.prog.mk to address "test binary, but not the
suite binary".

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D35604
MFC after: 2 weeks