This test has always been broken. The fix is not obvious, there was an attempt to set up mount namespaces, but this will fail if the output directory doesn't exist. In the default configuration it will fail because the directory doesn't exist until cross-seed runs the first time. This points at a real issue with the module that should be addressed. But for now let's remove the broken test, then we can add it back in working condition.
Original PR: https://github.com/NixOS/nixpkgs/pull/383409