test: add change prompt valid and invalid arg test

src/builtins/change_prompt.rs

@@ -48,4 +48,14 @@ mod tests {
     fn test_change_prompt_multiple_args() {
         assert_eq!(ChangePrompt.execute(&mut BuiltinConfig::new(), vec!["1".to_string(), "2".to_string()]), Err(ShellError::ExecuteFailure("expected only one argument".to_string())))
     }
+
+    #[test]
+    fn test_change_prompt_none() {
+        assert_eq!(ChangePrompt.execute(&mut BuiltinConfig::new(), vec!["none".to_string()]), Ok(()))
+    }
+
+    #[test]
+    fn test_change_prompt_invalid_style() {
+        assert_eq!(ChangePrompt.execute(&mut BuiltinConfig::new(), vec!["notexistent".to_string()]), Err(ShellError::ExecuteFailure("notexistent is not a valid prompt style".to_string())))
+    }
 }