How to test missing extra requirements?

On a python distribution (Plone related) that I'm developing I added an extra requirement: async

Thus, if someone wants to use it without async features that's fine, but they have also that possibility. Code has already been updated to use it conditionally.

But now the question is: on my test runner I declare my own distribution to install the async extra so I can test the async functionality, but that prevents me from testing the non-async code paths.

How can one handle this?