cue/testdata: add regression test for a cycle bug fixed in evalv3

As reported by Pete Heist, the following CUE used to work on v0.5:

    #A: {
        B?: #B
    }
    #B: {
        A: #A
    }

This is because the optional field breaks the cycle. However, v0.6
regressed and started giving a cycle error.

This bug is resolved in evalv3 thanks to its cycle detection algorithm.
Add a regression test to ensure it remains fixed.

Closes #2545.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I5ae9fcee08ea79d0ca76596dfc4ef71ecd2486fc
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1203434
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mvdan