netmap: Add some simple regression tests

[markj]

Summary

These test functionality of netmap's generic mode combined with several
types of software interfaces: currently just bridges, vlans and epairs.
To be added are tests for lagg and tap.

The test code includes a copy of bridge.c which has some functionality
removed and prints the number of packets forwarded in each direction.
This can be used to check that userspace indeed receives an expected
number of packets.

[vmaffione]

If useful, please note that there are already a bunch of integration tests in the github netmap repo, e.g.:
https://github.com/luigirizzo/netmap/blob/master/utils/randomized_tests

A short description here https://github.com/luigirizzo/netmap/blob/master/utils/README

[markj]

In D39431#897747, @vmaffione wrote:

If useful, please note that there are already a bunch of integration tests in the github netmap repo, e.g.:
https://github.com/luigirizzo/netmap/blob/master/utils/randomized_tests

A short description here https://github.com/luigirizzo/netmap/blob/master/utils/README

Oh, interesting. Do you think this could be integrated into FreeBSD's test suite without too much pain?

[vmaffione]

Sorry for the delay. I've little experience with the FreeBSD test suite, so I wouldn't know...