Add and xfail test for 2101

src/test/run-pass/issue-2101.rs

+// xfail-test
+use std;
+import std::arena;
+import std::arena::arena;
+
+enum hold { s(str) }
+
+fn init(ar: &a.arena::arena, str: str) -> &a.hold {
+    new(*ar) s(str)
+}
+
+fn main(args: [str]) {
+    let ar = arena::arena();
+    let leak = init(&ar, args[0]);
+    alt *leak {
+        s(astr) {
+            io::println(#fmt("%?", astr));
+        }
+    };
+}