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
All this is doing is letting you alias ``str(func())`` under another name.  Skip the middle man, and just call func() directly.  It's what...maybe 10 extra characters to type each time?  You don't need an entire new class to do something otherwise possible by a sequence of built ins.