tests: Add system-userspace-testsuite.

tests: Add system-userspace-testsuite.

The new system-userspace-testsuite, which can be launched via
`make check-system-userspace`, reuses the kmod tests on the userspace
datapath.

The userspace datapath is already tested by the main testsuite (and
that's not going to change), but having also the
system-userspace-testsuite has the following advantages:

* More complicated tests are possible: real client and server
  applications can be used.
* The same tests run on both kernel and userspace datapath: this gives
  us an easy way to make sure that the behaviour is consistent (e.g.
  with the upcoming connection tracker integration)

The userspace datapath is able to use system network interfaces via an
AF_PACKET socket.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joestringer@nicira.com>