Update p0f to version 3.0.7b, add test

- Update p0f to version 3.0.7b.
- Add a simple test in which p0f reads a pcap file.

In the test we write a simple binary pcap file (included in the recipe base64-encoded) and then attempt to read the file with p0f.

@brandt As long as the `.pcap` remains small I'm happy for it to be more useful. Could you try and update it and test that it works with the other formulae that are using it? Thanks :heart: