Write a tests developer's guide

We need to prepare documentation for test developers covering core things:

* Tests repository layout.
* Tests execution order.
* Ansible integration.
* Testinfra integration.
* BATS integration.
* How-to run existent tests like [CentOS functional tests](https://github.com/CentOS/sig-core-t_functional) or CloudLinux OS legacy tests?
* What output formats are supported for custom tests (are we going to support something except plain text output and TAP?)?
* How to include/exclude tests only for selected platforms and/or architectures?
* Provide some real world examples.