internal/core/adt: add tests for 3920

Issue #3920

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I32ba09143b97284cd24fe8ef78ee62ae835514bf
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1214870
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl