Motivation
We have a decorator @pytest.mark.usefixtures("_vary_network_conn") to allow us to test both online and offline. But there's currently no mechanism to ensure that this decorator is always used when it should be, which means there are tests which will fail when offline.

We should create a mechanism to run the test suite in offline mode in CI, if possible. Otherwise, we should just do a pass of running the test suite locally and making sure everything passes.

Summary of desired enhancement
Test that the test suite can pass when offline.