@Override
  public boolean mayRegisterTests() {
    return true;
  }
Maven Surefire removes nodes if {@link TestPlan#containsTests()} is <tt>false</tt>.
In order to avoid this, we return <tt>true</tt> here, because it makes {@link #containsTests(TestDescriptor)}
also return <tt>true</tt>, which in turn feeds into Maven's test plan.
This seems to work and not to bother Gradle either.

@return always <tt>true</tt> for error spec nodes
spock-core/src/main/java/org/spockframework/runtime/ErrorSpecNode.java