add tests

src/util.rs

@@ -3,3 +3,10 @@ use std::fs::remove_file;
 pub fn clean() {
     let _ignored = remove_file("temp");
 }
+
+#[test]
+fn test_clean() {
+    std::fs::File::create("temp").unwrap();
+    clean();
+    assert!(!std::path::Path::new("temp").exists());
+}