selftests: forwarding: mirror_gre_vlan_bridge_1q: Add more tests

Offloading of mirror-to-gretap in mlxsw is tricky especially in cases
when the gretap underlay involves bridges. Add more tests that exercise
the bridge handling code:

- forbidden_egress tests that check vlan removal on bridge port in the
  underlay packet path
- untagged_egress tests that similarly check "egress untagged"
- fdb_roaming tests that check whether learning FDB on a different port
  is reflected
- stp tests for handling port STP status of bridge egress port

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