Require a main function in all exercises

exercises/17_tests/tests2.rs

@@ -6,6 +6,10 @@
 // Execute `rustlings hint tests2` or use the `hint` watch subcommand for a
 // hint.
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     #[test]