how to add a unit test for the task?

@soloturn commented on [Sun Feb 24 2019](https://github.com/gradle-guides/writing-gradle-tasks/issues/14)

in former times, one could do for the unit test:
```
task.execute
```

can you please add a unit test for your hello-world task which checks it is working ok?

Locate the hello-world task and review how the guide currently demonstrates task execution, including the task.execute example. Determine where a unit test belongs and use it to verify that the hello-world task works correctly.

Written by the indexing model from the issue text.