cubicweb #4655613 __default_empty_db__ can be built with non-default ApptestConfiguration [open]

eg. if web/test/ is executed first, the cached default database is built with CubicWebServerConfig as configcls. This config cls redefine the default base url, so created entities have a different cwuri than the default "". This make web/test/ fails.

The default database should always be created with default configcls.

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