[9.x] Add withBasicAuth method

[stfndamjanovic]

This PR adds the possibility to easily attach a basic auth header in test cases. I think it can be useful when we are testing endpoints with basic auth.

So instead of this

$this->withToken(base64_encode("username:password"), "Basic")->get('/');

We can use this

$this->withBasicAuth('username', 'password')->get('/');