selftests: forwarding: mirror_gre_lib: Add STP test

Add a reusable full test that toggles STP state of a given bridge port
and checks that the mirroring reacts appropriately. The test will be
used by bridge tests in follow-up patches.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>