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

[takahiro.kurosawa_gmail.com]

Summary

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.

[kp]

Good point.