I found a simple solution by creating a static class that extends TransformerEndpointSpec and exposes a constructor:

@Test
public void buildTransformerSpecTest() {
    final Consumer<TransformerEndpointSpec> consumer = TransformerUtil.buildTransformerSpec();

    final MyTransformerEndpointSpec t = new MyTransformerEndpointSpec();
    consumer.accept(t);

    Assertions.assertEquals("myTransform", t.getId());
}

private static class MyTransformerEndpointSpec extends TransformerEndpointSpec { }