in maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/stubs/MavenProjectStub.java [326:332]
public void addCompileSourceRoot(String string) {
if (compileSourceRoots == null) {
compileSourceRoots = new ArrayList<>(Collections.singletonList(string));
} else {
compileSourceRoots.add(string);
}
}