* Support (BPF) filter in sniff() with offline parameter set
Fixes#393
Also, fixes#355
* Add tests for wrpcap() and sniff(offline=)
as suggested by Guillaume.
Also, cleanup regression.uts since it was a pain to find a place
to add those tests.
* Fix PATH for tcpdump with non-root user
* Do not run tcpdump tests when tcpdump is not available
* Appveyor tests: install WinDump.exe
Thanks @gpotter2