Add a can't-nest-rw-arcs test case (xfailed, #3177)

src/test/compile-fail/arc-cant-nest-rw-arc-3177.rs

@@ -0,0 +1,9 @@
+// xfail-test
+// error-pattern: instantiating a type parameter with an incompatible type
+use std;
+import std::arc::rw_arc;
+
+fn main() {
+    let arc1  = ~rw_arc(true);
+    let _arc2 = ~rw_arc(arc1);
+}