HttpTestTrait - Allow one to easily add Authx JWTs to each request

HttpTestTrait - Allow one to easily add Authx JWTs to each request

```php
// Add JWT credentials to every Guzzle request
$http = $this->createGuzzle([
  'authx_user' => 'admin',
]);
$response = $http->get('route://civicrm/dashboard');

// Add JWT credentials to a single request
$http = $this->createGuzzle();
$response = $http->get('route://civicrm/dashboard', [
  'authx_user' => 'demo',
]);

// Perform a stateful login via JWT
$http = $this->createGuzzle(['cookies' => new CookieJar()]);
$response = $http->get('route://civicrm/authx/login', [
  'authx_contact_id' => 100,
]);
```