pf tests: make synproxy and nat work correctly even if inetd is running

tests/sys/netfil/pf/synproxy fails if inetd has been running
outside of the jail because pidfile_open() fails with EEXIST.
tests/sys/netfil/pf/nat has the same problem but the test succeeds
because whether inetd is running is not so important.

Fix the problem by changing the pidfile path from the default
location.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29622