not testtools module on all build farm server

[tridge at samba.org]

Hi Matthieu,

 > ImportError: No module named testtools
 > Exit code was 1 How can we fix this ? build this module on our own ? 

We have a copy of testtools in lib/testtools already. The problem is
that source4/auth/credentials/tests/bind.py imported subunit/python
before it imported testtools, but subunit/python depends on testtools.

I've pushed a fix to autobuild.

Cheers, Tridge