Add a test function

add/add_one/src/lib.rs

@@ -3,3 +3,13 @@ use rand;
 pub fn add_one(x: i32) -> i32 {
     x + 1
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(3, add_one(2));
+    }
+}