Add more unit tests for allocation/balance

The patch adds some simple unit-tests for both the allocation function
(we can allocate small instances on an empty cluster, we can allocate in
tiered more starting from any size) and the balancing functions (one
single instance is placed optimally, a full cluster plus an empty node
can be rebalanced). The coverage has increased greatly, since this is
the bulk of the algorithm/code.

Also, the cluster tests are now being run with different options, since
they are much slower.