internal/core/adt: add tests for closedness

Issue #3325
Issue #3330
Issue #3331

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I16c3639fb016020ea58b75e7ecefa0fc1d1afe84
Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1199271
Reviewed-by: Matthew Sackman <matthew@cue.works>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>