internal/core/adt: add tests for 3858

Issue #3858

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I0f98ef0c425d060b9db59b247a57e342030cffc8
Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1213332
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl