Add an unwind test for failure during unique box construction

Author: brson

src/test/run-fail/unwind-partial-unique.rs

@@ -0,0 +1,18 @@
+// error-pattern:fail
+
+fn f() -> [int] { fail; }
+
+// Voodoo. In unwind-alt we had to do this to trigger the bug. Might
+// have been to do with memory allocation patterns.
+fn prime() {
+    @0;
+}
+
+fn partial() {
+    let x = ~f();
+}
+
+fn main() {
+    prime();
+    partial();
+}