Position components in KiCad for a test rig
...or perhaps some other easy way to guarantee that the tester PCB
will match with the testee

The way I'd do it is this: -

Save the design under a new name (say &quot;fixture&quot;)
Lock the mating components needed on the &quot;fixture&quot;
Delete all the components apart from those needed
Use this as the basis for your fixture design
Develop the fixture design but,
Make sure the component copper/pads/holes that align with the original PCB are position-locked on the fixture design.