selftests: loopback.sh: skip this test if the driver does not support

The loopback feature is only supported on a few drivers like broadcom,
mellanox, etc. The default veth driver has not supported it yet. To avoid
returning failed and making the runner feel confused, let's just skip
the test on drivers that not support loopback.

Fixes: ad113409 ("selftests: Add loopback test")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>