subtests.docker_cli: Add logs_follow test

This test uses `docker logs --follow` and checks it works correctly. (Issue https://github.com/autotest/autotest-docker/issues/250 )

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Docstring is suppose to describe **what** it is (the class), and **how** to use it. Regular comments descrbe **why** you decided to do something a certain way.  They fill in the one piece of information that's really difficult to get w/o lots of looking and guessing.  No worries though, writing good comments seems to take a lifetime to learn, I still fail at it quite frequently myself.