Fix MakesHttpRequests::call() return type (#1239)

Static analysis of a method using `call()` fails with the following error:

```
 ------ ----------------------------------------------------------------------------------------------------------------------------------------------------- 
  Line   src/Testing/MakesGraphQLRequests.php (in context of class Tests\Unit\Testing\TestingTraitDummy)
 ------ ----------------------------------------------------------------------------------------------------------------------------------------------------- 
  100    Method Tests\Unit\Testing\TestingTraitDummy::multipartGraphQL() should return Illuminate\Testing\TestResponse but returns Illuminate\Http\Response.
 ------ -----------------------------------------------------------------------------------------------------------------------------------------------------
```

src/Testing/Concerns/MakesHttpRequests.php

@@ -344,7 +344,7 @@ protected function formatToExpectedJson($key, $value)
      * @param  array  $files
      * @param  array  $server
      * @param  string  $content
-     * @return \Illuminate\Http\Response
+     * @return \Illuminate\Testing\TestResponse
      */
     public function call($method, $uri, $parameters = [], $cookies = [], $files = [], $server = [], $content = null)
     {