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

Here is the test result after conversion.

 # ./fib_tests.sh

 Single path route test
     Start point
     TEST: IPv4 fibmatch                                                 [ OK ]

 ...

 Fib6 garbage collection test
     TEST: ipv6 route garbage collection                                 [ OK ]

 IPv4 multipath list receive tests
     TEST: Multipath route hit ratio (1.00)                              [ OK ]

 IPv6 multipath list receive tests
     TEST: Multipath route hit ratio (1.00)                              [ OK ]

 Tests passed: 225
 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/20231213060856.4030084-13-liuhangbin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>