Add a test for const

Similar to the tests for Option and Result.

src/test/ui/consts/cow-is-borrowed.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(cow_is_borrowed)]
+
+use std::borrow::Cow;
+
+fn main() {
+    const COW: Cow<str> = Cow::Borrowed("moo");
+
+    const IS_BORROWED: bool = COW.is_borrowed();
+    assert!(IS_BORROWED);
+
+    const IS_OWNED: bool = COW.is_owned();
+    assert!(!IS_OWNED);
+}