Make demo tests disabled by default

Currently we're adding a non-macro demo function to each
unit test to facilitate debugging. This will be especially helpful
if a Renode or Qemu test fails, since one can step through easily while
looking for implementation bugs.

These however, need not be run during the test sequence, and added a note
describing their purplse and how to enable them for the debugging
circumstance.

Change-Id: I75a7e50154f7d7e3a509defe08cb6ae6722051cc