tests: fix a race condition in ftp server disconnect

If a client disconnected and reconnected quickly, before the ftp server
has a chance to respond, the protocol message/ack (ping/pong) sequence
got out of sync, causing messages sent to the old client to be delivered
to the new.  A disconnect must now be acknowledged and intermediate
requests thrown out until it is, which ensures that such synchronization
problems can't occur. This problem could affect ftp, pop3, imap and smtp
tests.

Fixes #12002
Closes #12049