This moves the generated root/ directory into /tmp, or at least a path
returned by tempfile.mkdtemp(), by default. This can make test runs
significantly faster if done when /tmp is a tmpfs.

If you are debugging a failed test, use the new --keep-root option to
not clean up and pactest will print the location of the generated root/
test directory.

Signed-off-by: Dan McGee <dan@archlinux.org>