[10.x] Fix in appendExceptionToException method exception type check (#…

…49958)

* Fix in appendExceptionToException method exception type check

* Update TestResponse.php

---------

Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Testing/TestResponse.php

@@ -1651,7 +1651,7 @@ public function transformNotSuccessfulException($exception)
      */
     protected function appendExceptionToException($exceptionToAppend, $exception)
     {
-        $exceptionMessage = $exceptionToAppend->getMessage();
+        $exceptionMessage = is_string($exceptionToAppend) ? $exceptionToAppend : $exceptionToAppend->getMessage();
 
         $exceptionToAppend = (string) $exceptionToAppend;