`local -a stuff` does not make `stuff` a bound array variable, so
checking length of the array will trigger an error about unbound
variable. Fortunately, `local stuff=()` does the trick.
We forgot to clear the array with instance tests to rerun, so the list
grew from one iteration to another when going over all the instance
types. I did not spot it before, because I tested it with only one
extra instance.
