@Override
  public void start(CoprocessorEnvironment env) throws IOException {
    this.env = (RegionCoprocessorEnvironment) env;
    this.seenRegion = new ConcurrentHashMap<>();
  }
Mocks the behavior of getMin() in the real AggregateImplementation. Makes some fake data to
send back some partial results, and then finally a complete result.
hbase-endpoint/src/test/java/org/apache/hadoop/hbase/client/MockPartialResultAggregateImplemention.java