The ssisUnit GUI does not support creating the persisted dataset. If you switch the IsResultsStored flag to true on the dataset’s properties, it gives a warning “The expected dataset’s (<the dataset name>) stored results does not contain data. Populate the Results data table before executing.” during the test run.
To find out more about it, take a look at the source code.
This is a nice explanation of a current limitation in the tool and a workaround.