selftests/net: convert fib-onlink-tests.sh to run it in unique namespace

Remove PEER_CMD, which is not used in this test

Here is the test result after conversion.

 ]# ./fib-onlink-tests.sh
 Error: ipv4: FIB table does not exist.
 Flush terminated
 Error: ipv6: FIB table does not exist.
 Flush terminated

 ########################################
 Configuring interfaces

   ...

     TEST: Gateway resolves to wrong nexthop device - VRF      [ OK ]

 Tests passed:  38
 Tests failed:   0

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-11-liuhangbin@gmail.com


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