ftp: Disable ipv6 support

Clients use the extended passive and port commands even when using ipv4 (RFC
2428 requires them to do so). Since redirected ftp is not supported with the
extended commands, the addition of ipv6 support forced proxy mode for all
transfers when using ipv6 capable clients.

Until we resolve this issue, this patch disables the ipv6 related unit tests.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: none