tests: kernel: mem_protect: use correct define for MEM_REGION_ALLOC

For ARM architecture, use Z_THREAD_MIN_STACK_ALIGN to define
MEM_REGION_ALLOC in tests/kernel/mem_protect/mem_protect/.
STACK_ALIGN takes into account MPU stack guard alignment
requirements. However, application memory partitions do not
require MPU stack guards, therefore, the alignment requirements
are not applicable here.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>