cmd/cue: add test case for issue 3250
`@if` tags are currently treated as false when they're inside
a non-package file inside a module. Add a test case
for this, to be fixed in a subsequent CL.
For #3250.
Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I1aafec01ce2586158488bb30d77506622e53eaee
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202737
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>