ERP5TypeLiveTestCase: repair running live tests from zope

This fixes a regression with 445e8fa8 (ERP5TypeTestCase: rework
default manager password generation, 2024-05-27), it was no longer
possible to run live tests from a "normal" zope instance, this was
failing with:

Traceback (most recent call last):
  File "./eggs/Zope-5.10-py3.9.egg/Testing/ZopeTestCase/PortalTestCase.py", line 59, in setUp
    self._setup()
  File "./parts/erp5/product/ERP5Type/tests/ERP5TypeLiveTestCase.py", line 156, in _setup
    self.login()
  File "./parts/erp5/product/ERP5Type/tests/ERP5TypeTestCase.py", line 290, in login
    return PortalTestCase.login(self, user_name)
  File "./eggs/Zope-5.10-py3.9.egg/Testing/ZopeTestCase/PortalTestCase.py", line 146, in login
    user = user.__of__(uf)
AttributeError: 'NoneType' object has no attribute '__of__'

It was only possible from a runUnitTest instance.