From whatwg/html#5009 (comment), by @annevk:

One other thing is that label can be activated and this causes the control it points towards to also match :active. Given how that's currently defined (one definition of active is mouse down) that should always work and would be testable.

This originally came up in the context of form-associated custom elements but I don't believe they're special in this regard; this is a general test that could be written for any labeled control.