test/smtp-dummy: add --background option and functionality

When shell executes background process using '&' the scheduling of
that new process is arbitrary. It could be that smtp-dummy doesn't
get execution time to listen() it's server socket until some other
process attempts to connect() to it. The --background option in
smtp-dummy makes it to go background *after* it started to listen
its server socket.

When --background option is used, the line "smtp_dummy_pid='<pid>'"
is printed to stdout from where shell can eval it.