[6.x] Adds assertJsonPath to TestResponse

[ecrmnn]

In our test we want to assert the following:

• The post has no tags
• The first comment belongs to the user ecrmnn
• The body of the first comment is First! and the body of the second comment is This is my comment

Given the following response from our Post API:

{
  "title": "My blog post",
  "body": "Lorem ipsum ...",
  "tags": [],
  "comments": [{
  	"body": "First!",
  	"user_id": 42,
  	"user": {
  		"id": 42,
  		"username": "ecrmnn"
  	}
  }, {
  	"body": "This is my comment",
  	"user_id": 731,
  	"user": {
  		"id": 731,
  		"username": "ventrec"
  	}
  }]
}

The problem

Using assertEquals

- Forces us to create a temporary variable
- Assertions are not chainable

$response = $this->getJson(route('api.post.show', [$post->id]));

$this->assertEmpty($response->json('tags'));
$this->assertEquals('ecrmnn', $response->json('comments.0.user.username'));
$this->assertEquals([
    'First!',
    'This is my comment',
], $response->json('comments.*.body'));

Using assertExactJson

- Can not ignore irrelevant data
- Forces us to provide the entire response to make three simple assertions

$this->getJson(route('api.post.show', [$post->id]))
	->assertExactJson([
	    'title' => 'My blog post',
	    'body' => 'Lorem ipsum ...',
	    'tags' => [],
	    'comments' => [
	        [
	            'body' => 'First!',
	            'user_id' => 42,
	            'user' => [
	                'id' => 42,
	                'username' => 'ecrmnn',
	            ],
	        ],
	        [
	            'body' => 'This is my comment',
	            'user_id' => 731,
	            'user' => [
	                'id' => 731,
	                'username' => 'ventrec',
	            ],
	        ],
	    ],
	]);

Using assertJsonFragment

- We don't know if the first or the second user is the one named ecrmnn
- Unable to assert on the body of the first and second comment

$this->getJson(route('api.post.show', [$post->id]))
	->assertJsonFragment([
	    'tags' => [],
	    'username' => 'ecrmnn',
	]);

The solution

This PR introduces assertJsonPath in the TestResponse class. It makes it possible to assert that a given value exists at the specified path.

Using assertJsonPath

+ No need for a temporary variable
+ Lets us assert on a specific user
+ No need to include irrelevant data in the assertion

$this->getJson(route('api.post.show', [$post->id]))
    ->assertJsonPath('tags', [])
    ->assertJsonPath('comments.0.user.username', 'ecrmnn')
    ->assertJsonPath('comments.*.body', [
        'First!',
        'This is my comment',
    ]);