[5.8] Make TestResponse tappable

[SjorsO]

This PR adds the new tappable trait to the TestResponse. This way you can do stuff like this:

$this->getJson('/api/users/index')
    ->assertStatus(200)
    ->tap(function (TestResponse $response) {
        $this->assertMatchesJsonSnapshot($response);
    });

Instead of having to do:

$response = $this->getJson('/api/users/index')
    ->assertStatus(200);

$this->assertMatchesJsonSnapshot($response);