[Merge] lp:~laney/ubuntu-release-upgrader/testfix into lp:ubuntu-release-upgrader

[Julian Andres Klode]

Review: Approve

LGTM. 

I was thinking about making this more generic, but this requires a few tricks, but should fully isolate the rootdir, possibly by adding an argument to apt.Cache, like:

apt.Cache(..., config={})

to start a cache with an empty config. Needs changes in python-apt, update-manager, and ubuntu-release-upgrader, but should make things nice. What do you think?
-- 
https://code.launchpad.net/~laney/ubuntu-release-upgrader/testfix/+merge/341842
Your team Ubuntu Core Development Team is requested to review the proposed merge of lp:~laney/ubuntu-release-upgrader/testfix into lp:ubuntu-release-upgrader.