Create test for `dest_folder`
Create test for `dest_folder`

This commit creates a test for the `dest_folder` class parameter. Without it,
Lint tests fail, and we don't get a nice big green glowing bulb in Jenkins. Of
course, it's also much better to have tests. So this, erm, tests this.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet