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

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

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Link: https://lore.kernel.org/r/20240603093322.3150030-1-lukma@denx.de


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