internal/core/adt: add tests to prepare for bug fix

Note that the results for "embed" are different from
the results of "direct", even though they should not be.
The results of "direct" are correct.

This fix is in a follow-up CL.

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ifa2801a2566309a17b0cd3bcf3a193125b0286f9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200327
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>