internal/core/adt: add tests for Issue 3474

Validators are not correctly evaluated when combining
embedding with comprehensions.

Issue #3474

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I62cc860dec7515c91e470efa71cfa8b28bc0ea9b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202119
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>