tests/lib/heap: Add coverage for sys_heap_usable_size()

Call and validate the new function.  Note that this is actually
whiteboxing the heap implementation and duplicating the internal logic
needed to compute the header size, so as to validate that the correct
number is being returned.  I had to write that test code anyway, might
as well commit it (the heap header is awfully well optimized now and
very unlikely to change).

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