As we have strong belief in Drupal community I would like to offer some free accounts for testing Drupal projects. Our tool is great for testing things like themes or distributions.
Umami team uses us a lot and already creating tickets with screenshots from our UI.
As an example check https://www.drupal.org/project/drupal/issues/2954378#comment-13325411.
The easiest way to start would be to use https://simplytest.me to create two environments: one is baseline and another one the one with patch you are trying to test. Then with Diffy you can compare them and see the difference.
If you are interested in using Diffy for your projects please reach out to firstname.lastname@example.org and I will be happy to get you on board