Use tempfile.mkdtemp to create temp dirs for tests

I prefer to have test dirs below /tmp which is a tmpfs on most Debian
installations.

test_PristineTar.py also had code to create a tmpdir. I had to add a call to
gbp.log.setup because somehow I got debug log output which made the tests fail
(see below):

File "<SNIP>/git-buildpackage/tests/test_PristineTar.py", line 33, in test_create
Failed example:
    repo = gbp.deb.git.DebianGitRepository.create(repo_dir)
Expected nothing
Got:
    gbp:debug: git ['init'] []
    gbp:debug: ['git', 'rev-parse', '--show-cdup']
    gbp:debug: ['git', 'rev-parse', '--is-bare-repository']
redirect stdout to sys.stdout in command_wrappers

Thus nosetest will capture the output of failed tests (only) and
show it together with failed tests.
Before that, output was mixed with the progress points or letters
of the testrunner.