Add test cases

issue-912 is xfailed. issue-2111 is already fixed, but it's good to
have the test case.

src/test/compile-fail/issue-2111.rs

@@ -0,0 +1,11 @@
+fn foo(a: option<uint>, b: option<uint>) {
+  alt (a,b) { //! ERROR: non-exhaustive patterns: none not covered
+    (some(a), some(b)) if a == b { }
+    (some(_), none) |
+    (none, some(_)) { }
+  }
+}
+
+fn main() {
+  foo(none, none);
+}