internal/core/adt: add tests for 3934

Issue #3934

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ied07624986390071efa3bd29e34d4a57410dc3f2
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1215544
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl