selftests: forwarding: new test, verify host mdb entries

Boiler plate for testing static mdb entries.  This first test verifies
adding and removing host mdb entries for all supported types: IPv4,
IPv6, and MAC multicast.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>