internal/core/adt: fix structural cycle error in list.Sum

The sum in the changed test was summing itself, which
is a structural cycle. The old behavior was to give
a garbage result. We now correctly report the structural
cycle.

Issue cue-lang#3310

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ia37a3a0adf373e9455dc921916a1352ebd580b44
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202444
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl