test: add env variable resolve test

src/preprocess.rs

@@ -41,4 +41,15 @@ mod tests {
         assert_eq!(preprocess("key=value".to_string()), Ok(None));
         assert_eq!(env::var("key").unwrap_or_default(), "value".to_string())
     }
+
+    #[test]
+    fn test_preprocessing_resolve_env_variable() {
+        assert_eq!(preprocess("key=value".to_string()), Ok(None));
+        assert_eq!(preprocess("$key".to_string()), Ok(Some("value".to_string())))
+    }
+
+    #[test]
+    fn test_preprocessing_resolve_non_existent_env_variable() {
+        assert_eq!(preprocess("$nonexitent".to_string()), Ok(Some("".to_string())))
+    }
 }