Add initial solution files for Rustlings exercises and quizzes

- Created empty solution files for various exercises in strings, modules, hashmaps, options, error handling, generics, traits, lifetimes, tests, iterators, smart pointers, threads, macros, clippy, conversions, and quizzes.
- Each solution file contains a main function with a comment indicating that it will be automatically filled after completing the exercise.
- Added a README.md file to provide information about the solutions and their purpose.

exercises/17_tests/tests3.rs

@@ -0,0 +1,51 @@
+struct Rectangle {
+    width: i32,
+    height: i32,
+}
+
+impl Rectangle {
+    // Don't change this function.
+    fn new(width: i32, height: i32) -> Self {
+        if width <= 0 || height <= 0 {
+            // Returning a `Result` would be better here. But we want to learn
+            // how to test functions that can panic.
+            panic!("Rectangle width and height must be positive");
+        }
+
+        Rectangle { width, height }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn correct_width_and_height() {
+        // TODO: This test should check if the rectangle has the size that we
+        // pass to its constructor.
+        let rect = Rectangle::new(10, 20);
+        assert_eq!(rect.width, 10); // Check width
+        assert_eq!(rect.height, 20); // Check height
+    }
+
+    // TODO: This test should check if the program panics when we try to create
+    // a rectangle with negative width.
+    #[test]
+    #[should_panic]
+    fn negative_width() {
+        let _rect = Rectangle::new(-10, 10);
+    }
+
+    // TODO: This test should check if the program panics when we try to create
+    // a rectangle with negative height.
+    #[test]
+    #[should_panic]
+    fn negative_height() {
+        let _rect = Rectangle::new(10, -10);
+    }
+}