No need to test file without permission.

src/main.rs

@@ -67,7 +67,6 @@ mod tests {
     fn test_open_file() {
         assert!(open_file("two_robots.txt").is_ok());
         assert!(open_file("test_unexisting_file.extension").is_err());
-        assert!(open_file("no_permission.txt").is_err());
     }
 
 }