internal/core/adt: add tests for 3964

Issue #3964

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ib99a0d3ef253617cd8ba71be7759a2dd1e6fd57e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1216870
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl