selftests: tc_actions: test egress 2nd vlan push

Add new test checking the correctness of inner vlan flushing to the skb
data when outer vlan tag is added through act_vlan on egress.

Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>