Add basic unit test for keys

In preparation for modifying the `keys` function add a basic unit test.

Author: tcharding

src/policy/concrete.rs

@@ -1222,4 +1222,14 @@ mod tests {
 
         assert_eq!(got, want);
     }
+
+    #[test]
+    fn keys() {
+        let policy = Policy::<String>::from_str("or(and(pk(A),pk(B)),pk(C))").unwrap();
+
+        let want = vec!["A", "B", "C"];
+        let got = policy.keys();
+
+        assert_eq!(got, want);
+    }
 }