In rare cases where Message-IDs get reused, we do not want to
hold onto the large Email::MIME objects in memory after showing
the first message. So discard each message as soon as we're
done using it so we can save memory for the next message.
The new and expensive xt/mem-msgview.t test shows a nearly 14MB
reduction for two ~7MB messages. run_script() also gets
upgraded to make it easier to pass large inputs via IO GLOBs.