test: hsr: Extend the hsr_redbox.sh to have more SAN devices connected

After this change the single SAN device (ns3eth1) is now replaced with
two SAN devices - respectively ns4eth1 and ns5eth1.

It is possible to extend this script to have more SAN devices connected
by adding them to ns3br1 bridge.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Link: https://lore.kernel.org/r/20240510143710.3916631-1-lukma@denx.de


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