test/system: fix "podman stop - unlock while waiting for timeout"

In highly parallel runs this can fail its timeout due the fact that we
run podman ps -a which takes all contianer locks, even the ones from
unrelated tests. thus this command can take a long time, from a CI log I
see:
[18:22:30.932958589] # /var/tmp/podman/bin/podman  ps -a
[18:22:34.338904713] CONTAINER ID  IMAGE

So like a 4s delay just on that and hence the test later failed with
"Operations took too long" as it took 7 seconds overall.

So since we know podman inspect takes the lock just keep that and not
try to get all containers for no reason.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>