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>
Dispatch-Trailer: {"type":"trybot","CL":1202872,"patchset":1,"ref":"refs/changes/72/1202872/1","targetBranch":"release-branch.v0.10"}

cmd/cue/cmd/testdata/script/issue3250.txtar

@@ -0,0 +1,21 @@
+# This checks that @if tags work for files specified on the command line.
+
+# passes - no module
+exec cue export -t foo .:foo
+exec cue export -t foo foo.cue
+
+# set up module
+exec cue mod init
+
+# passes - with module
+exec cue export -t foo .:foo
+
+# fails - with module
+# TODO fix this case
+! exec cue export -t foo foo.cue
+
+-- foo.cue --
+@if(foo)
+package foo
+
+x: 1