selftests: net: fib-onlink-tests: Convert to use namespaces by default

selftests: net: fib-onlink-tests: Convert to use namespaces by default

Currently, the test breaks if the SUT already has a default route
configured for IPv6. Fix by avoiding the use of the default namespace.

Fixes: 4ed591c8ab44 ("net/ipv6: Allow onlink routes to have a device mismatch if it is the default route")
Suggested-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260113-selftests-net-fib-onlink-v2-1-89de2b931389@suse.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>