ZEOServerTestCase: use .close() instead of .close_server()

This was a deprecated method that no longer exists since ZEO 4
https://github.com/zopefoundation/ZEO/commit/5423f110ddc1034ea0954495957894c60f6da78c

product/ERP5Type/tests/ERP5TypeTestCase.py

@@ -1575,7 +1575,7 @@ class ZEOServerTestCase(ERP5TypeTestCase):
       pass
 
   def tearDown(self):
-    self.zeo_server.close_server()
+    self.zeo_server.close()
 
 
 class lazy_func_prop(object):