return $this for chaining (#55060)

looks like a small oversight from #54678 of which part of it was caught in #54941

Author: browner12

src/Illuminate/Testing/TestResponse.php

@@ -994,6 +994,8 @@ public function assertOnlyJsonValidationErrors($errors, $responseKey = 'errors')
         $unexpectedErrorKeys = Arr::except($jsonErrors, $expectedErrorKeys);
 
         PHPUnit::withResponse($this)->assertTrue(count($unexpectedErrorKeys) === 0, 'Response has unexpected validation errors: '.collect($unexpectedErrorKeys)->keys()->map(fn ($key) => "'{$key}'")->join(', '));
+
+        return $this;
     }
 
     /**