netpfil tests: improve sniffer.py
Multiple improvements to sniffer.py:
- Remove ambiguity of configuring recvif, it must be now explicitly specified.
- Don't catch exceptions around creating the sniffer, let it properly fail and display the whole stack trace.
- Count correct packets so that duplicates can be found.
MFC after: 1 week
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D38120
(cherry picked from commit a39dedeb31052ec74b0cd394d56f8d7cc8534645)