selftests: mlxsw: Add mlxsw-specific test for mirror to gretap

Test several aspects of offloading mirror to gretap and ip6gretap
netdevices that are specific to mlxsw, such as requirements for TTL and
TOS values.

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