tests: fix cache issue causing random failure in testAuthenticationPolicy

Traceback (most recent call last):
  File "Products/ERP5/tests/testAuthenticationPolicy.py", line 538, in test_05_HttpRequest
    self.assertFalse(person.isLoginBlocked())
  File "Products/ERP5/mixin/login_account_provider.py", line 68, in isLoginBlocked
    return method(**kw)
  File "ZODB/Connection.py", line 857, in setstate
    raise ConnectionStateError(msg)
ConnectionStateError: Shouldn't load state for 0x6c90 when the connection is closed

product/ERP5Type/tests/ERP5TypeTestCase.py

@@ -695,6 +695,10 @@ class ERP5TypeTestCaseMixin(ProcessingNodeTestCase, PortalTestCase):
         # Restore security manager
         setSecurityManager(sm)
 
+        # Make sure that the skin cache does not have objects that were
+        # loaded with the connection used by the requested url.
+        self.changeSkin(self.portal.getCurrentSkinName())
+
         return ResponseWrapper(response, outstream, path)
 
     def getConsistencyMessageList(self, obj):