test: Add a test for vector reference counts, XFAIL'd in rustc

Author: pcwalton

src/test/run-pass/vec-ref-count.rs

@@ -0,0 +1,11 @@
+// xfail-stage0
+
+use std;
+import std._vec;
+
+fn main() {
+    auto v = vec(1, 2, 3);
+    check (_vec.refcount[int](v) == 1u);
+    check (_vec.refcount[int](v) == 1u);
+}
+