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

Test that it's possible to offload a given number of mirrors.

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