cue: add regression test for issue 3042

In helping Richard on Slack with a use case needing a recursive schema,
we noticed that one of the constraints in the schema wasn't applying
properly when the data used a few levels of nesting.

Luckily, the new evaluator already seems to handle this well,
erroring as expected where the old evaluator incorrectly succeeds.
Add a regression test to ensure that the new evaluator doesn't regress.
Given that we never spotted this bug in the old evaluator before,
we clearly did not have any equivalent or close enough test case.

The issue can now be closed with the regression test.
Given that all work is focused on the new evaluator, it doesn't seem
worthwhile to spend the time to backport a fix into the old evaluator.

Closes #3042.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I964776fe6057bccf74ab6ae7546fdf2b74d2420b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194086
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mvdan