selftests: hsr: Extend the hsr_redbox.sh test to use fixed MAC addresses

Fixed MAC addresses help with debugging as last four bytes identify the
network namespace.

Moreover, it allows to mimic the real life setup with for example bridge
having the same MAC address on each port.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Link: https://lore.kernel.org/r/20240603093322.3150030-2-lukma@denx.de


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