ftp: fix unit tests so they run on IPv4-only machine

The unit tests for the AbstractFtpDoor are, on the most part,
designed to run independently of whether the testing machine has
IPv6 support or not.

Unfortunately one test relied on the host machine having the IPv6
loop-back interface enabled.  This patch fixes that problem.

Target: master
Request: 2.8
Request: 2.7
Patch: http://rb.dcache.org/r/6882/
Acked-by: Gerd Behrmann