subtests.docker_cli: Add run_user test

this test changes --user and verifies it works correctly.

I encounter the problem with non-exiting containers unless stdin/stdout is used. I solved it by writing `\n` to stdin to avoid test hang and add FIXME note there. Probably we should put higher priority to this bug...

This patch fixes the https://github.com/autotest/autotest-docker/issues/209