in maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/stubs/MavenProjectStub.java [336:342]
public void addScriptSourceRoot(String string) {
if (scriptSourceRoots == null) {
scriptSourceRoots = new ArrayList<>(Collections.singletonList(string));
} else {
scriptSourceRoots.add(string);
}
}