Bug#754143: Fix the tests to not pollute the data directory

[Raphaël Hertzog <hertzog@debian.org>]

Package: tracker.debian.org
Severity: minor

After a run of "./manage.py test" you get a few files in the data
directory:
data/cache: 4a1eaab869bd498dd2218b580f58a891
data/cache: 4a1eaab869bd498dd2218b580f58a891.headers
data/cache: 78016d49ce86f47b39d5712970ca07cf
data/cache: 78016d49ce86f47b39d5712970ca07cf.headers
data/cache: 99ce8528c1922aba6057158485a10285
data/cache: 99ce8528c1922aba6057158485a10285.headers
data/cache: apt-cache
data/cache: fabdbad731826600bd98665400c59da6
data/cache: fabdbad731826600bd98665400c59da6.headers
data/keyring: pubring.gpg
data/keyring: secring.gpg

The tests should always work on temporary directories and not on directories
meant for use in production. We want to be able to run tests on our production
server without risking the integrity of our data.

-- System Information:
Debian Release: jessie/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing'), (500, 'stable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.14-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=fr_FR.utf8, LC_CTYPE=fr_FR.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash