subtests.docker_cli: Add run_env test

This test contains workaround using InteractiveAsyncDockerCmd due of https://bugzilla.redhat.com/show_bug.cgi?id=1131592 and it fixes the issue https://github.com/autotest/autotest-docker/issues/252

this tests the ENV related features. Currently it only contains one
subsubtest, which is focused on linked containers. It sends data
using the addr from ENV to the server, which resends them back.

In the end it checks the valid data + ENV variables added via `-e x=y`
including the linked values ($linked_ENV_...)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Right, sorry, I'm mixing up things.  I mean this ``InteractiveAsyncDockerCmd`` pattern is used frequently, workaround or not.  In addition, we could use it as a central place to fuss with the final '\n' issue (for now) instead of putting it in every test.  How about adding a comment and final sending of '\n' inside ``close()``?  Maybe tie it to a ``if self.config.get('trailing_n_workaround', False)`` and update to ``defaults.ini``?

I'm not that interested in solving the "finish-discovery" issue this class, that kind of thing is better handled by the caller.