[5.8] Allow TestResponse dump method chaining

[derekmd]

#28960 merged today no longer exits PHPUnit when calling dump() nor dumpHeaders(). However method chaining for the typical HTTP feature test won't work.

This change allows the test to continue as normal, showing PHPUnit's test case Pass/Fail along with the TestResponse dump.

Example dump() chaining

$this->getJson(route('posts.show', $post))
    ->dump() // now returns null
    ->assertStatus(Response::HTTP_UNAUTHORIZED)
    ->assertJson([
        'error' => 'token_not_provided',
    ]);

Before

  Feature
    Tests\Feature\PostsTest
        testShowDeniesAuthorizationToGuest        {#8568
  +"status": "success"
}
ERROR
     
Error: Call to a member function assertStatus() on null

After

  Feature
    Tests\Feature\PostsTest
        testShowDeniesAuthorizationToGuest       {#8568
  +"status": "success"
}
FAIL
    
Response status code [200] does not match expected 401 status code.
Failed asserting that false is true.