@Test
  public void testWALObserverRoll() throws Exception {
    final WAL wal = wals.getWAL(null);
    final SampleRegionWALCoprocessor cp = getCoprocessor(wal, SampleRegionWALCoprocessor.class);
    cp.setTestValues(TEST_TABLE, null, null, null, null, null, null, null);

    assertFalse(cp.isPreWALRollCalled());
    assertFalse(cp.isPostWALRollCalled());

    wal.rollWriter(true);
    assertTrue(cp.isPreWALRollCalled());
    assertTrue(cp.isPostWALRollCalled());
  }
Test to see CP loaded successfully or not. There is a duplication at TestHLog, but the purpose
of that one is to see whether the loaded CP will impact existing WAL tests or not.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestWALObserver.java