Added `test_open_file()`

src/main.rs

@@ -62,4 +62,12 @@ mod tests {
         assert!(parse_instruction('F').is_ok());
         assert!(parse_instruction('Z').is_err());
     }
+
+    #[test]
+    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());
+    }
+
 }