Every one of us knows the case when you need to add more class names to style your component depending on some conditions, but you do not want to have negative boolean values in className prop.
We had the same issue.
So, what are you going to do if you need to solve a problem like this?
We at Kontentino are testing our UI components with unit testing and we use with functional testing for our utility methods. But when it came to testing our UI with data asynchronously fetched from the server we just needed to find out the way how to do that simple and with maximum effectivity. We researched the web and found out that there are plenty of articles about how you could test your components with async functions. Web articles are usually recommending to fake your backend API with some tools or some dummy functions on the client-side. This approach is according to us good but it requires some amount of configuring and programming dummy code which costs additional time.