test/test-lib.sh: take the --background feature in smtp-dummy into use

test/test-lib.sh: take the --background feature in smtp-dummy into use

The use of --background option (instead of shell '&') ensures that
smtp-dummy is listening its server socket until execution of shell
script can continue, thus the client will always have socket where
to connect.

smtp-dummy outputs smtp_dummy_pid variable in shell assignment format;
eval'ing that output makes that variable available for the shell.

As the smtp-dummy instance is no longer child process of the script
the SIGKILL signal sent to it will ensure it is going away in case
the mail sender fails to connect to smtp-dummy.