netpfil tests: Move pft_ping.py and sniffer.py to the common test directory

The pft_ping.py and sniffer.py tool is moved from tests/sys/netpfil/pf to
tests/sys/netpfil/common directory because these tools are to be used in
common for all the firewalls.

Submitted by: Ahsan Barkati
Reviewed by: kp, thj
Sponsored by: Google, Inc. (GSoC 2019)
Differential Revision: https://reviews.freebsd.org/D21276