Add some exercises about tests!

tests/tests3.rs

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+// 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 `is_even(5)`. Scroll down for hints!
+
+pub fn is_even(num: i32) -> bool {
+    num % 2 == 0
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_true_when_even() {
+        assert!(false);
+    }
+}
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+// You can call a function right where you're passing arguments to `assert!` -- so you could do
+// something like `assert!(having_fun())`. If you want to check that you indeed get false, you
+// can negate the result of what you're doing using `!`, like `assert!(!having_fun())`.