internal/core/adt: add tests for 3590 and 3591

The issue is a regression introduced in
https://cuelang.org/cl/1199752

Issue #3590
Issue #3591

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I1e2a9be08a7957b72c4904856f68af8f9efc3c45
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1206278
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>