[6.x] Add assertJsonPath

[ahinkle]

Adds the new validation rule, assertJsonPath.

laravel/framework#29957

[taylorotwell]

Could you flesh out the example a bit so it's clearer what you should be passing?

[ahinkle]

Will do. To keep the Available Assertions section uniform, I've added an example within the Testing JSON APIs section. Let me know if I need to provide any additional examples.

[dailytabs]

The documentation for this is pathetic. Standard json path notation starts with $. This is not supported and not documented. Array access is impossible to figure out. I just need to test that the first array item has a certain property and value.
[0].name is null
$[0].name is null
$.[0].name is null
.name is null
..name is null

I can dump my response and it's right there! 40 times in this array! Plain as day: "name": "value"

Like seriously, any explanation of WTF $path is would be...ya know, proper documentation!

EDIT: Found this comment on SO and thought it was funny: "6.3.3: throws an exception that decodeResponseJson() has been deprecated. Would check the doc but we all know how pointless that is..."

[ahinkle]

Feel free to open a new PR; suggesting that your entitled comment of "pathetic" doesn't help anyone.

[driesvints]

@dailytabs can you please keep your comments more civil? Your remarks aren't helping anyone.