I want to write an integration test that confirms that two different polylith projects A and B speak to one another correctly. What is the recommended pattern here? I was thinking about creating a third project C that depends upon bases A and B, but this feels a little unidiomatic.
It depends on what you want to test, and how close to the production environment the test needs to be. If you only want to test the logic, then you could try something similar to what you suggest, where bricks talk directly to bricks, but if you need running services, then you have to set it up according to that.