add tests; refactor exercise links

exercises/tests/tests1.rs

@@ -1,7 +1,7 @@
 // tests1.rs
 // Tests are important to ensure that your code does what you think it should do.
 // Tests can be run on this file with the following command:
-// rustc --test tests1.rs
+// rustlings run --test exercises/tests/tests1.rs
 
 // This test has a problem with it -- make the test compile! Make the test
 // pass! Make the test fail! Scroll down for hints :)

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!();
     }
 }
 

exercises/tests/tests4.rs

@@ -1,19 +0,0 @@
-// tests4.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
-// we expect to get when we call `times_two` with a negative number.
-// No hints, you can do this :)
-
-pub fn times_two(num: i32) -> i32 {
-    num * 2
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn returns_twice_of_positive_numbers() {
-        assert_eq!(4, 4);
-    }
-}