maven-archetype-archetype/src/main/resources/archetype-resources/src/main/resources/META-INF/maven/archetype.xml

<archetype>
  <id>${artifactId}</id>
  <sources>
    <source>src/main/java/App.java</source>
  </sources>
  <testSources>
    <source>src/test/java/AppTest.java</source>
  </testSources>
</archetype>