test: add preprocess set variable test

src/preprocess.rs

@@ -22,3 +22,14 @@ pub fn preprocess(line: String) -> Result<Option<String>, ShellError> {
 
     Ok(Some(line))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_preprocess_env_variable() {
+        assert_eq!(preprocess("key=value".to_string()), Ok(None));
+        assert_eq!(env::var("key").unwrap_or_default(), "value".to_string())
+    }
+}