[devtools] Fix database creation issues for CubicWebServerTC tests
To reproduce (with a purged repo):
* python web/test/unittest_web.py MiscOptionsTC
* python web/test/unittest_views_xmlrss.py
The core issue is that CubicWebServerConfig.default_base_url() is called
inside init_config(), which is called during the equivalent of
'db-create'. The '__default_empty_db__' will contain the wrong
'base_url' configuration, thus breaking URLs in the xmlrss tests.
Related to #9297611.