samples: drivers: mbox: switch nrf54h20 cpuapp and cpurad remotes

Switch which board is the remote in the test case for
mbox communication between nrf54h20dk/nrf54h20/cpuapp and
nrf54h20dk/nrf54h20/cpurad, making cpurad the remote instead.
This is done to prepare the sample for executing with IronSide SE,
where using cpurad as the main board doesn't make as much sense,
since cpuapp has to start cpurad.

Signed-off-by: Jonathan Nilsen <jonathan.nilsen@nordicsemi.no>