Add test for uint and negative literals as discriminants

Author: kemurphy

src/test/run-pass/enum-discr.rs

@@ -0,0 +1,20 @@
+enum Animal {
+    Cat = 0u,
+    Dog = 1u,
+    Horse = 2u,
+    Snake = 3u
+}
+
+enum Hero {
+    Batman = -1,
+    Superman = -2,
+    Ironman = -3,
+    Spiderman = -4
+}
+
+fn main() {
+    let pet: Animal = Snake;
+    let hero: Hero = Superman;
+    assert!(pet as uint == 3);
+    assert!(hero as int == -2);
+}