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

Here is the test result after conversion.

2 tests also failed without this patch

]# ./arp_ndisc_untracked_subnets.sh
    TEST: test_arp:  accept_arp=0                                       [ OK ]
    TEST: test_arp:  accept_arp=1                                       [ OK ]
    TEST: test_arp:  accept_arp=2  same_subnet=0                        [ OK ]
    TEST: test_arp:  accept_arp=2  same_subnet=1                        [ OK ]
    TEST: test_ndisc:  accept_untracked_na=0                            [ OK ]
    TEST: test_ndisc:  accept_untracked_na=1                            [ OK ]
    TEST: test_ndisc:  accept_untracked_na=2  same_subnet=0             [ OK ]
    TEST: test_ndisc:  accept_untracked_na=2  same_subnet=1             [ OK ]

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