protected TreeMap<ServerName, List<RegionInfo>> mockUniformClusterServers(int[] mockCluster) {
    int numServers = mockCluster.length;
    TreeMap<ServerName, List<RegionInfo>> servers = new TreeMap<>();
    for (int i = 0; i < numServers; i++) {
      int numRegions = mockCluster[i];
      ServerAndLoad sal = randomServer(0);
      List<RegionInfo> regions = uniformRegions(numRegions);
      servers.put(sal.getServerName(), regions);
    }
    return servers;
  }
This assumes the RegionPlan HSI instances are the same ones in the map, so actually no need to
even pass in the map, but I think it's clearer.
@return a list of all added {@link ServerAndLoad} values.
hbase-balancer/src/test/java/org/apache/hadoop/hbase/master/balancer/BalancerTestBase.java