The mock setup in the old test was very limited, the Node structs it set
up were left to zeros, and even allocated with incorrect lengths (SubPlan
vs SubPlanState). It worked just enough for the codepath that it was
testing, but IMHO it's better to test the error "in vivo", and it requires
less setup, too. So remove the mock test, and replace with a fault
injector test that exercises the same codepath.