protected Map<ServerName, List<RegionInfo>> mockClusterServers() throws IOException {
    assertTrue(servers.size() == regionAssignment.length);
    Map<ServerName, List<RegionInfo>> assignment = new TreeMap<>();
    for (int i = 0; i < servers.size(); i++) {
      int numRegions = regionAssignment[i];
      List<RegionInfo> regions = assignedRegions(numRegions, servers.get(i));
      assignment.put(servers.get(i), regions);
    }
    return assignment;
  }
Asserts a valid retained assignment plan.
<p>
Must meet the following conditions:
<ul>
<li>Every input region has an assignment, and to an online server
<li>If a region had an existing assignment to a server with the same address a a currently
online server, it will be assigned to it
</ul>
hbase-server/src/test/java/org/apache/hadoop/hbase/master/balancer/RSGroupableBalancerTestBase.java