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
Based on (typically bad) experiences, ``__del__()`` has nearly always eventually caused much more trouble than benefit.  It tends to be useful/helpful most of the time, however when it's not, then things go really horribly wrong.  Strange as it sounds, It's normally better to just let the file-descriptor leak than try to find where it was accidentally closed.  In other words omitting explicit behavior (i.e. a ``close()`` call) is easier to detect than an implicit, hidden call.