Write a tests developer's guide

[ezamriy]

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 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.