tests(tcp) reduce flakiness using a handshake in tcp_server
tests(tcp) reduce flakiness using a handshake in tcp_server

Add the same kind of handshake in `helpers.tcp_server` that we already do in
`helpers.udp_server`. Hopefully, this will reduce flakiness in tests that use
the `tcp_server` such as `tcp-log`.

Kong/kong,Mashape/kong,Kong/kong,Kong/kong