subtests.docker_cli: Add detach and interaction test

This pull request implements the https://github.com/autotest/autotest-docker/pull/192 inside the test instead of framework to give it time to stabilize before turning it into framework. I believe that this implementation is cleaner and easier to follow than hybrid implementation from https://github.com/autotest/autotest-docker/pull/194

Kind regards,
Lukáš
Suggestion:  It's just a small touch, but sometimes really helpful:  Have this method return something, for example ``self.cmdresult`` or ``self.exit_status``.  Normally would probably just be discarded, but when it's needed would be super handy.