Add some exercises about tests!

tests/tests4.rs

@@ -0,0 +1,18 @@
+// 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);
+    }
+}