cmd/cue: add test cases for issue 3968
We add test cases for the panic described in issue 3968,
to be fixed in a subsequent CL.
For #3968.
Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I2a4968f09d1cdcadfbb232f0f021f3027a63a858
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1216861
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>