[TST-E9] OS-only test subjects run on their OS, and the roster proves it #496

Part of #350, the quality-refactoring roadmap (road to 1.0, step 3).

Goal: OS-only test subjects run on their OS, and the roster proves it.

Lane: Tests. Status: todo.

Every PR of this epic starts its title with [TST-E9]. The finding issues it closes are linked as sub-issues; this issue closes when the epic's last PR merges and master CI is green.