Add an example of internal unit testing

adder/src/lib.rs

@@ -1,5 +1,9 @@
 pub fn add_two(a: usize) -> usize {
-    a + 2
+    internal_adder(a, 2)
+}
+
+fn internal_adder(left: usize, right: usize) -> usize {
+    left + right
 }
 
 #[cfg(test)]
@@ -7,8 +11,8 @@ mod tests {
     use super::*;
 
     #[test]
-    fn it_adds_two() {
-        let result = add_two(2);
+    fn internal() {
+        let result = internal_adder(2, 2);
         assert_eq!(result, 4);
     }
 }