Require a main function in all exercises

exercises/17_tests/tests1.rs

@@ -10,6 +10,10 @@
 // Execute `rustlings hint tests1` or use the `hint` watch subcommand for a
 // hint.
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     #[test]

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]

exercises/17_tests/tests3.rs

@@ -11,6 +11,10 @@ pub fn is_even(num: i32) -> bool {
     num % 2 == 0
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

exercises/17_tests/tests4.rs

@@ -7,7 +7,7 @@
 
 struct Rectangle {
     width: i32,
-    height: i32
+    height: i32,
 }
 
 impl Rectangle {
@@ -16,10 +16,14 @@ impl Rectangle {
         if width <= 0 || height <= 0 {
             panic!("Rectangle width and height cannot be negative!")
         }
-        Rectangle {width, height}
+        Rectangle { width, height }
     }
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;