Added unit tests for RebalancePlan, RebalanceBatchPlan, and AbstractZonedRebalanceTest.

Added following rebalance test case coverage:
- RebalancePlanTest / RebalanceBatchPlanTest / AbstractZonedRebalanceTest
- no-op / shuffle / cluster expansion / zone expansion
- 2-zone / 3-zone

Also did some clean up of rebalance test utils and rebalance tests in general.