ztest: add comment about expected thread priorities

There is an implicit expectation that the priority of threads spawned by
a ztest suite is still higher than the main suite thread own priority
when a test signals the main suite thread that it has completed.

Change-Id: Id7caec3e9e553712c828a93c212b8e82bd16eabd
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>