subtests.docker_cli.kill: Update the test to work with latest docker

Latest docker doesn't return the exit code, but follows this schema:
* with tty=on `docker kill` => 0
* with tty=off `docker kill` => 255
* bash `kill -9 $cont_pid` => 137

Skip the exit code check for now and wait until the behavior stabilizes.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>