Improve the prop_Alloc_sane test to detect mis-allocations

Currently, this just checks that a cluster cannot be rebalanced after
a single instance allocation. However, we can also test whether the
allocation decision computed a correct new cluster score, by checking
that against the one computed from the actual new node list.

Also, for nicer display, we convert the test from a Boolean to a
Property, with nice annotations.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>