Add a test that assignment of unique boxes to locals does a copy

Issue #409

src/test/run-pass/unique-assign-copy.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let i = ~mutable 1;
+    // Should be a copy
+    let j;
+    j = i;
+    *i = 2;
+    *j = 3;
+    assert *i == 2;
+    assert *j == 3;
+}