[11.x] new: ddJson method on TestResponse class

[chester-sykes]

This PR adds a ddJson() method to the TestResponse class. This is a dd() wrapper around the existing json() which would be very handy if you just want to quickly see what the json response is when writing tests.

Instead of doing this:

$response = $this->getJson('/api/test-api');

dd($response->json());

you could just do this:

$this->getJson('/api/test-api')->ddJson();

I wasn't sure it was possible to write a test for this because the function uses dd() but if it is possible could someone please explain how to do it and I'll add a test.

The benefit of this is to reduce the time it takes to see the result of the response when testing, instead of having to use a variable. it won't break any existing logic as its a new function.