Changed the generated test script to use a part-local tmp directory

that is cleaned up at the start of a test run.

Unfortunately, the ZODB tests leave lots of temporary files behind
which can cause failures in subsequent test runs and which tend to
litter /tmp. Eventually, I want to clean that up, but, in the mean
time, I can limit the damage to the test part directory.

buildout.cfg

@@ -6,6 +6,11 @@ find-links = http://download.zope.org/distribution/
 [test]
 recipe = zc.recipe.testrunner
 eggs = ZODB3
+initialization = 
+  import os, tempfile, shutil
+  if os.path.exists('tmp'): shutil.rmtree('tmp')
+  os.mkdir('tmp')
+  tempfile.tempdir = os.path.abspath('tmp')
 
 [scripts]
 recipe = zc.recipe.egg