add tests; refactor exercise links

exercises/tests/tests3.rs

@@ -1,6 +1,6 @@
 // tests3.rs
 // This test isn't testing our function -- make it do that in such a way that
-// the test passes. Then write a second test that tests that we get the result
+// the test passes. Then write a second test that tests whether we get the result
 // we expect to get when we call `is_even(5)`. Scroll down for hints!
 
 pub fn is_even(num: i32) -> bool {
@@ -13,7 +13,7 @@ mod tests {
 
     #[test]
     fn is_true_when_even() {
-        assert!(false);
+        assert!();
     }
 }