test: Add a test for generic objects. r=test-only

src/test/run-pass/generic-object.rs

@@ -0,0 +1,20 @@
+trait Foo<T> {
+    fn get() -> T;
+}
+
+struct S {
+    x: int
+}
+
+impl S : Foo<int> {
+    fn get() -> int {
+        self.x
+    }
+}
+
+fn main() {
+    let x = @S { x: 1 };
+    let y = x as @Foo<int>;
+    assert y.get() == 1;
+}
+