@Test
  public void testScanPriority() throws Exception {
    testForCall(new ThrowingCallable() {
      @Override
      public void call() throws IOException {
        ResultScanner scanner = TABLE.getScanner(new Scan());
        scanner.next();
      }
    });
  }
Tests that one can implement their own RpcControllerFactory and expect it to successfully pass
custom priority values to the server for all HTable calls.
hbase-server/src/test/java/org/apache/hadoop/hbase/TestCustomPriorityRpcControllerFactory.java