tc-tests: actions: add MPLS tests

Add a new series of selftests to verify the functionality of act_mpls in
TC.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/tc-testing/config

@@ -42,6 +42,7 @@ CONFIG_NET_ACT_CTINFO=m
 CONFIG_NET_ACT_SKBMOD=m
 CONFIG_NET_ACT_IFE=m
 CONFIG_NET_ACT_TUNNEL_KEY=m
+CONFIG_NET_ACT_MPLS=m
 CONFIG_NET_IFE_SKBMARK=m
 CONFIG_NET_IFE_SKBPRIO=m
 CONFIG_NET_IFE_SKBTCINDEX=m