selftests/net: use unique netns name for setup_loopback.sh setup_veth.sh

The setup_loopback and setup_veth use their own way to create namespace.
So let's just re-define server_ns/client_ns to unique name.
At the same time update the namespace name in gro.sh and toeplitz.sh.
As I don't have env to run toeplitz.sh. Here is only the gro test result.

 # ./gro.sh
 running test ipv4 data
 Expected {200 }, Total 1 packets
 Received {200 }, Total 1 packets.
 ...
 Gro::large test passed.
 All Tests Succeeded!

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