internal/core/eval: verify issue 299 if fixed

Fixes #299

Change-Id: I3a45b40b6273294185e8ba99bbfd61dfb9422d65
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7046
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/testdata/builtins/issue299.txtar

@@ -0,0 +1,31 @@
+-- in.cue --
+import "list"
+x: list.UniqueItems
+x: [string, ...string]
+x: ["x","x"]
+-- out/eval --
+Errors:
+x: invalid value ["x","x"] (does not satisfy list.UniqueItems)
+
+Result:
+(_|_){
+  // [eval]
+  x: (_|_){
+    // [eval] x: invalid value ["x","x"] (does not satisfy list.UniqueItems)
+    0: (string){ "x" }
+    1: (string){ "x" }
+  }
+}
+-- out/compile --
+--- in.cue
+{
+  x: 〈import;list〉.UniqueItems
+  x: [
+    string,
+    ...string,
+  ]
+  x: [
+    "x",
+    "x",
+  ]
+}