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

Here is the test result after conversion.

]# ./ndisc_unsolicited_na_test.sh
    TEST: test_unsolicited_na:  drop_unsolicited_na=0  accept_untracked_na=1  forwarding=1  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=0  accept_untracked_na=0  forwarding=0  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=0  accept_untracked_na=0  forwarding=1  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=0  accept_untracked_na=1  forwarding=0  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=1  accept_untracked_na=0  forwarding=0  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=1  accept_untracked_na=0  forwarding=1  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=1  accept_untracked_na=1  forwarding=0  [ OK ]
    TEST: test_unsolicited_na:  drop_unsolicited_na=1  accept_untracked_na=1  forwarding=1  [ OK ]

Tests passed:   8
Tests failed:   0

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