selftests/net: convert netns-name.sh to run it in unique namespace

This test will move the device to netns 1. Add a new test_ns to do this.
Here is the test result after conversion.

 # ./netns-name.sh
 netns-name.sh                           [  OK  ]

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>