in maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/stubs/MavenProjectStub.java [346:352]
public void addTestCompileSourceRoot(String string) {
if (testCompileSourceRoots == null) {
testCompileSourceRoots = new ArrayList<>(Collections.singletonList(string));
} else {
testCompileSourceRoots.add(string);
}
}