selftests: forwarding: Test mirror-to-gretap w/ UL 802.1d

Test for "tc action mirred egress mirror" that mirrors to gretap when
the underlay route points at a VLAN-unaware bridge (802.1d).

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