Add some exercises about tests!

tests/tests1.rs

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+// This test has a problem with it -- make the test compile! Make the test
+// pass! Make the test fail! Scroll down for hints :)
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn you_can_assert() {
+        assert!();
+    }
+}
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+// You don't even need to write any code to test -- you can just test values and run that, even
+// though you wouldn't do that in real life :) `assert!` is a macro that needs an argument.
+// Depending on the value of the argument, `assert!` will do nothing (in which case the test will
+// pass) or `assert!` will panic (in which case the test will fail). So try giving different values
+// to `assert!` and see which ones compile, which ones pass, and which ones fail :)