internal/core/adt: add tests for Issue 3929

This got fixed somewhere along the way.

Fixes #3929

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I2b7486445208aabc4e462ae66291c2134d5c1bff
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217128
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl