netpfil tests: Improve sniffer.py

[vegeta_tuxpowered.net]

Summary

Part 1 of 5 of tests for D38025

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.

Sponsored by: InnoGames GmbH

[kp]

I think we're missing a change of foundCorrectPacket to correctPackets in pft_ping.py here.

[kp]

Also, minor nit, but can you include the full file for context in future diffs?

No need to update this one, but it does make reviews easier. Mark's git-arc.sh (./tools/tools/git/git-arc.sh) usually does the right thing, and tends to make Phabricator/git interactions a little less painful.

[vegeta_tuxpowered.net]

In D38120#866233, @kp wrote:

I think we're missing a change of foundCorrectPacket to correctPackets in pft_ping.py here.

Ineed that part was missing. I've split the changes into separate commits wrongly. I've uploaded the updated patch.