@After
  public void tearDown() throws Exception {
    if (this.bc != null) this.bc.shutdown();
  }
Set up variables and get BucketCache and WriterThread into state where tests can manually
control the running of WriterThread and BucketCache is empty.
hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/bucket/TestBucketWriterThread.java