test/system: fix "podman kill - concurrent stop"

It is possible that the background podman stop -t 1 completes before we
get to podman kill making the kill fail as the container is not running.

The result is this error in the test:
"Error: can only kill running containers. xxx is in state exited: container state improper"

We can just bump the stop timeout to have a larger time where we can
trigger kill on a stopping container.

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