Maybe I'm missing something here, but I'm extending the nunjucks transformer to be able to handle async rendering as well. For this I added a renderAsync method that handles this.
I also added a test case that mimics an async template loader.
This test case (obviously) fails with the sync methods, as it should. But there's no way to exclude the methods from being tested on the async test case.
The other way around (testing the async method on sync test cases) isn't a problem, since the renderAsync handles them perfectly.