Fix flaky test in DecommissionWorkerSuite

The test `decommission workers ensure that shuffle output is regenerated even with shuffle service` assumes it has two executor and both of two tasks can execute concurrently.

The two tasks will execute serially if there only one executor. The result is test is unexpceted. E.g. 

```

[info] 5 did not equal 4 Expected 4 tasks but got List(0:0:0:0-SUCCESS, 0:0:1:0-FAILED, 0:0:1:1-SUCCESS, 0:1:0:0-SUCCESS, 1:0:0:0-SUCCESS) (DecommissionWorkerSuite.scala:190)

```