lib/rbtree: Fix & document insert comparison order

The scheduler priq implementation was taking advantage of a subtle
behavior of the way the tree presents the order of its arguments (the
node being inserted is always first).  But it turns out the tree got
that wrong in one spot.

As this was subtle voodoo to begin with, it should have been
documented first.  Similarly add a little code to the test case to
guarantee this in the future.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>