sleftests/net: convert icmp.sh to run it in unique namespace

Here is the test result after conversion.

]# ./icmp.sh
OK

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>