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