Adding description of how to test a command which expects user input

This PR adds a description on testing a command which uses the `DialogHelper` and expects some user input.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.0+
| Fixed tickets | #2351

One thing I'm not sure about is if I should add a link to the section about command testing. If so, where should I add it?