selftests/net: convert test_bridge_neigh_suppress.sh to run it in unique namespace

Here is the test result after conversion.

]# ./test_bridge_neigh_suppress.sh

Per-port ARP suppression - VLAN 10
----------------------------------
TEST: arping                                                        [ OK ]
TEST: ARP suppression                                               [ OK ]

...

TEST: NS suppression (VLAN 20)                                      [ OK ]

Tests passed: 148
Tests failed:   0
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>