Clean up buildout-created log handlers

In an attempt prevent extra logging in text output.  I'm not sure if
this results in less garbage output than we had before. It definately
reduces spurious output from the new getting-started test.

In any case, these should be cleaned up.

src/zc/buildout/testing.py

@@ -212,6 +212,9 @@ def buildoutSetUp(test):
             root_logger.removeHandler(handler)
         for handler in handlers_before_set_up:
             root_logger.addHandler(handler)
+        bo_logger = logging.getLogger('zc.buildout')
+        for handler in bo_logger.handlers[:]:
+            bo_logger.removeHandler(handler)
     register_teardown(restore_root_logger_handlers)
 
     base = tempfile.mkdtemp('buildoutSetUp')