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

Here is the test result after conversion.

 ]# ./fib_nexthop_nongw.sh
 TEST: nexthop: get route with nexthop without gw                    [ OK ]
 TEST: nexthop: ping through nexthop without gw                      [ OK ]

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-9-liuhangbin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>