cue/testdata: add test cases for #3924

Reduced test cases from cuelang.org/docs content show that
edge cases involving ellipsis and matchN no longer fail with
closedness errors as we expect them to.

For #3924.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ia0bae2c8c97f4d5b4bd6dbcbf50d64685163f9a9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1215073
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mvdan