cmd/cue: add tests for issue 1926

Closes #1926

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: If5968e3a8aef8b116daea71268bc4349f8207f5b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/549134
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/testdata/script/vet_issue1926.txtar

@@ -0,0 +1,38 @@
+# https://cuelang.org/issues/1926
+
+exec cue export foo.cue dev.json
+exec cue vet    foo.cue dev.json
+
+exec cue export foo.cue prd.json
+exec cue vet    foo.cue prd.json
+
+-- foo.cue --
+package foo
+
+_env: =~"(prd)-"
+
+#Foo
+
+#Foo: {
+	env: "dev" | _env
+	baz: bool | *false
+
+	if env == "dev" {
+		_defaults.dev
+	}
+}
+
+_defaults: {
+	"dev": {
+		baz: true
+	}
+}
+
+-- prd.json --
+{
+	"env": "prd-foo"
+}
+-- dev.json --
+{
+	"env": "dev"
+}