Ping: Make build reproducible

As recently fixed in sockstat (9934558460e4), having tests/Makefile
include files from the parent directory with SRCS= ../foo.c results
in a race condition as the parent build and the tests build try to
produce the same object file but contain different paths.

Use .PATH to tell make to find sockstat.c in the parent directory
but place the object file in the current object directory.

Approved by: re (cperciva)
Reviewed by: emaste, jrtc27, kevans
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53075

(cherry picked from commit 3c9a2f383fc25cdffa80c1348cb5388290e0f283)
(cherry picked from commit a7c1d6b72a3e3b979d38ad1ad706cc0d9be927ae)