Reduce the number of workers

It is for reducing resources for a test.

It is better the the current configuration because all parameters use
different values like the following:

  * The number of workers is 4.
  * The number of slices is 3.
  * The number of replications is 2.

But it requires 12 worker processes (= 4 * 3 * 2) for a test. It is
resource overuse.