[10.x] Adds testing helpers for Precognition

[peterfox]

This should help anyone wanting to write a test for an endpoint using Precognition.

$this->usingPrecognition()
    ->get('/')
    ->assertPrecognitionSuccess();

[timacdonald]

This is awesome, Peter. I've put a few suggestions in. Tomorrow I'll come in and do a good review to make sure it's good to go.

Tried to today, but my Git foo failed me...so unfortunately I just added a bunch of GitHub comments instead of just making the changes - sorry!

Let's leave this in review until I've had a chance to go over it in detail.

Thanks again. Very excited to see this.

[peterfox]

@timacdonald thanks for reviewing! I've made the changes bar one that I left a comment on about asserting the response is a 204 status code.

[timacdonald]

Thanks, Peter!