@Override
    protected void reduce(ImmutableBytesWritable key, Iterable<ImmutableBytesWritable> values,
      Context context) throws IOException, InterruptedException {
      makeAssertions(key, values);
    }
Checks the last and first keys seen against the scanner boundaries.
hbase-mapreduce/src/test/java/org/apache/hadoop/hbase/mapreduce/MultiTableInputFormatTestBase.java