Make a tmpdir macro available that points at a temporary directory
which will be cleaned up after the test has run.  Adjust test cases to
use this rather than hard-coding paths in /tmp.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4455 d4fa192b-c00b-0410-8231-f00ffab90ce4