erp5_core_test testERP5Catalog: Do not compare creation_date with modificatoin_date

Creation date is defined as the oldest date of any workflow history on the
considered document.
Modification date is defined as the newest date of any workflow history
on the considered document.
As soon as there is more than one workflow on a given document, these
values can differ: they are set with the wall-clock time at the moment
the workflow history of each workflow is initially filled, so one can land
just before a second change and the other right after.
So comparing them is meaningless, catalog or not, and just causes this test
to be unstable.

bt5/erp5_core_test/TestTemplateItem/portal_components/test.erp5.testERP5Catalog.py

@@ -609,8 +609,6 @@ class TestERP5Catalog(ERP5TypeTestCase, LogInterceptor):
                       result[0]['creation_date'].ISO())
     self.assertEqual(modification_date,
                       result[0]['modification_date'].ISO())
-    self.assertEqual(creation_date,
-                      result[0]['modification_date'].ISO())
 
     import time; time.sleep(3)
     organisation.edit(title='edited')