selftests: rtnetlink: use setup_ns in bonding test

selftests: rtnetlink: use setup_ns in bonding test

This is a follow-up of commit a159cbe81d3b ("selftests: rtnetlink: check
enslaving iface in a bond") after the merge of net-next into net.

The goal is to follow the new convention,
see commit d3b6b1116127 ("selftests/net: convert rtnetlink.sh to run it in
unique namespace") for more details.

Let's use also the generic dummy name instead of defining a new one.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240115135922.3662648-1-nicolas.dichtel@6wind.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>