Add new test for checking multi-allocations

This test expands the "single-alloc-no-rebalance" by allocating a few
instances on a small cluster, and ensuring that after we allocate all
of them, either we can't rebalance or if we rebalance the score
improvement is very small.

The last condition is needed because sometime rounding errors (we're
using double-precision floating point) can accumulate and result in
what is a no real change in the cluster state, but with an
infinitesimal score decrease (e.g. 1e-14).

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