Change tests for pattern-matching exercise (#2463)

to catch wrong `right == 0` checks.

To change the number like this catches for example a wrong
implementation only looking if `right_result` is zero, but not checking
if we are doing a division.

src/pattern-matching/exercise.rs

@@ -111,6 +111,34 @@ fn test_recursion() {
     );
 }
 
+#[test]
+fn test_zeros() {
+    assert_eq!(
+        eval(Expression::Op {
+            op: Operation::Add,
+            left: Box::new(Expression::Value(0)),
+            right: Box::new(Expression::Value(0))
+        }),
+        Ok(0)
+    );
+    assert_eq!(
+        eval(Expression::Op {
+            op: Operation::Mul,
+            left: Box::new(Expression::Value(0)),
+            right: Box::new(Expression::Value(0))
+        }),
+        Ok(0)
+    );
+    assert_eq!(
+        eval(Expression::Op {
+            op: Operation::Sub,
+            left: Box::new(Expression::Value(0)),
+            right: Box::new(Expression::Value(0))
+        }),
+        Ok(0)
+    );
+}
+
 #[test]
 fn test_error() {
     assert_eq!(