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

Here is the test result after conversion.

 ]# ./fib_nexthops.sh

 Basic functional tests
 ----------------------
 TEST: List with nothing defined                                     [ OK ]
 TEST: Nexthop get on non-existent id                                [ OK ]

 ...

 TEST: IPv6 resilient nexthop group torture test                     [ OK ]

 Tests passed: 234
 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>
Link: https://lore.kernel.org/r/20231213060856.4030084-10-liuhangbin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>