tests: mem_protect: use MP_NUM_CPUS instead of SMP

The test_mem_domain_migration test creates a new thread with
different priority based on whether SMP is enabled. This causes
an issue where SMP=y and MP_NUM_CPUS=1 where the spin_entry()
would spin forever (with k_busy_wait()) and not yielding since
it has cooperative priority. Fix this by using MP_NUM_CPUS to
figure out which priority to use, as it is valid configuration
to have SMP=y and MP_NUM_CPUS=1.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>