Since test:///default resets state on every connection, writing a test
that covers a sequence of commands must be done from a single
session. But if the test wants to exercise particular failure modes as
well as successes, it can be nice to leave witnesses in the stderr
stream immediately before and after the spot where the expected error
should be, to ensure the rest of the script is not causing errors.

Do this by adding an --err option.
Signed-off-by: NEric Blake <eblake@redhat.com>
Acked-by: NMichal Privoznik <mprivozn@redhat.com>