selftests: rtnetlink: check enslaving iface in a bond

The goal is to check the following two sequences:
> ip link set dummy0 up
> ip link set dummy0 master bond0 down

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240108094103.2001224-3-nicolas.dichtel@6wind.com


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