@Test
  public void testWALObserverWriteToWAL() throws Exception {
    final WAL log = wals.getWAL(null);
    verifyWritesSeen(log, getCoprocessor(log, SampleRegionWALCoprocessor.class), false);
  }
Test WAL write behavior with WALObserver. The coprocessor monitors a WALEdit written to WAL,
and ignore, modify, and add KeyValue's for the WALEdit.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestWALObserver.java