Support (BPF) filter in sniff() with offline parameter set (#394)

* 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