internal/core/adt: add tests for 3921

Issue #3921

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I0960165cca5826c1ee7547e0e6909d1733cd107e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1214895
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl