Add an example use of the assert_eq macro

adder/src/lib.rs

@@ -1,5 +1,5 @@
-pub fn add(left: u64, right: u64) -> u64 {
-    left + right
+pub fn add_two(a: usize) -> usize {
+    a + 2
 }
 
 #[cfg(test)]
@@ -7,13 +7,8 @@ mod tests {
     use super::*;
 
     #[test]
-    fn exploration() {
-        let result = add(2, 2);
+    fn it_adds_two() {
+        let result = add_two(2);
         assert_eq!(result, 4);
     }
-
-    #[test]
-    fn another() {
-        panic!("Make this test fail");
-    }
 }