SB archetype doesn't include DMN example with a test when decisions/predictions starters are specified. However, for processes a BPMN example with a test class is included. The issue is the following condition in the archetype-post-generate.groovy:

if (!starters.contains("decisions") || !starters.contains("predictions") || !starters.contains("processes")) { // remove DMN example

The last condition is always satisfied.