internal/core/adt: add tests for #4015

Issue #4015

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I261e72b18c6f55e6c85c8e65efe0da711945e40f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1219789
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1220062
Reviewed-by: Roger Peppe <[email protected]>

Author: mpvl