qa: use the SO_REUSEADDR flag in functional tests' WSGI server

It was possible that the previous server socket did not have enough time
to expire, which could cause an "Address already in use" exception.

product/ERP5Type/tests/ProcessingNodeTestCase.py

@@ -151,7 +151,9 @@ class ProcessingNodeTestCase(ZopeTestCase.TestCase):
           for ip, port in parseListeningAddress(zserver):
             s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
             try:
+              s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
               s.bind((ip, port))
+              s.listen(0)
             except socket.error as e:
               s.close()
               if e[0] != errno.EADDRINUSE: