selftests: forwarding: Add IPv6 GRE flat tests

Add tests that check IPv6-in-IPv6, IPv4-in-IPv6 and MTU change of GRE
tunnel. The tests use flat model - overlay and underlay share the same VRF.

These tests can be run with TC_FLAG=skip_sw, so then they will verify
that packets go through hardware as part of enacp and decap phases.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>