[8.x] Add dd() method to TestResponse class

[duncanmcclean]

This PR adds a new dd method to the fluent TestResponse class.

Example

Previously... you'd have to comment out any assertions at the end of your chain, assign it to a variable and finally actually dd it.

$response = $this
  ->get('/posts');
//  ->assertOk()
//  ->assertSee('foo');

dd($response);

Now... with this PR, you'll be able to simply add the ->dd() method anywhere in your chain and the HTTP response will be die dumped.

$this
  ->get('/posts')
  ->dd()
  ->assertOk()
  ->assertSee('foo');

---

I've reviewed the dd methods available in other places in the codebase, like in the Builder class and the Stringable class and I couldn't find any tests that cover the dd method so I was unsure on how to actually add tests for this addition. Please let me know if I've missed anything.

[GrahamCampbell]

You should probably have this instead call the dump method.

Also, I think this will make a mess for parallel testing... // cc @nunomaduro

[derekmd]

You should probably have this instead call the dump method.

A TestResponse@dump() method already exists:

framework/src/Illuminate/Testing/TestResponse.php

Lines 1077 to 1095 in fe72373

	/**
	* Dump the content from the response.
	*
	* @return $this
	*/
	public function dump()
	{
	$content = $this->getContent();
	$json = json_decode($content);
	if (json_last_error() === JSON_ERROR_NONE) {
	$content = $json;
	}
	dump($content);
	return $this;
	}

That method originally called dd() but it was changed to dump() to allow database teardown, PHPUnit script cleanup, etc.

To short-circuit a test being dumped, maybe throwing a PHPUnit\Framework\SkippedTestError exception is more appropriate instead of calling dd() which immediately exits the test runner.

[taylorotwell]

I would just call dump() and run that single test.