bpo-35602: Make sure the transport is always closed in SelectorEventLoopUnixSockSendf...

bpo-35602: Make sure the transport is always closed in SelectorEventLoopUnixSockSendfileTests (GH-11338)

There is a race condition in SelectorEventLoopUnixSockSendfileTests that causes the prepare() method return a non connected server protocol, making the cleanup() method skips the correct handling of the transport. This commit makes prepare() always return a connected server protocol that can always be cleaned up correctly.
(cherry picked from commit d51324a2f5d172665f8824b25456c9822797fc84)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>