Add a test to confirm references are getting parsed.

src/parser/parser.rs

@@ -711,6 +711,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_reference_to_variable() {
+        assert_eq!(
+            super::reference("{$idx}"),
+            Ok((
+                "",
+                Reference {
+                    path: Path { keys: vec!["$idx"] },
+                    filters: Vec::new(),
+                }
+            ))
+        );
+    }
+
     #[test]
     fn test_path() {
         assert_eq!(