htools: fix prop_Node_rMem corner case

This patch fixes a bug in the test specification where we allowed nodes
with zero free memory (hence no instance can be added, at all) and adds
a simple labeling of the way this test can fail.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>