Changes

testenv: podman.exec_cmd: better error if stopped

Testenv may try to run a comand in podman after the container was
stopped, if there is a bug in the shutdown logic. Give a meaningful
error in that case, instead of failing later on with a cryptic error in
subprocess.run() because None was passed inside cmd (for the container
name) instead of a string.

Change-Id: I29ebe200258a9a9ffb127ec8fa1260b7e2124c38