tests: zbus/hlp_priority_boost: use 1cpu setup

The zbus hlp_priority_boost test requires thread priority
boosting. If it is running SMP, it is possible that
the message has already been processed before the check
for priority boosting is done, thus failing the test.
So change the test to use 1 CPU only.

Fixes #79197

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