add tests for `is_valid()` method

src/main.rs

@@ -294,4 +294,27 @@ mod tests {
         w.create();
         assert!(w.empty_position(Position { x: 0, y: 0 }));
     }
+
+    #[test]
+    fn test_valid_position() {
+        let p: Position = Position { x: 1, y: 1 };
+        let w: World = World {
+            x: 5,
+            y: 5,
+            map: Vec::new(),
+        };
+        assert!(p.is_valid(w).is_ok());
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_invalid_position() {
+        let p: Position = Position { x: -1, y: 1 };
+        let w: World = World {
+            x: 5,
+            y: 5,
+            map: Vec::new(),
+        };
+        assert!(p.is_valid(w).is_ok());
+    }
 }