protected List<ServerAndLoad> randomServers(int numServers, int numRegionsPerServer) {
    List<ServerAndLoad> servers = new ArrayList<>(numServers);
    for (int i = 0; i < numServers; i++) {
      servers.add(randomServer(numRegionsPerServer));
    }
    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