selftests: forwarding: mirror_gre_bridge_1d_vlan: Add STP test

To test offloading of mirror-to-gretap in mlxsw for cases that a
VLAN-unaware bridge is in underlay packet path, test that the STP status
of bridge egress port is reflected.

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