internal/core/adt: add tests for Issue 4000

Issue #4000

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I1e50302bf165e002149d6cb33c570d7605e36f5e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1219236
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl