[testlib] call init_config once the config has been properly bootstraped This should be done right after repository instantiation and as such involves giving CWTC.init_config as a callback to the test database handler (which will itself gives it to the repository initialization function). This unfortunatly requires to pass the init_config method to the server.init_repository function because it has to be called after the config has been initialized (which is done in Repository.__init__) but before the migration handler is instantiated (which will call 'server_maintainance' hook, hence may require the proper config). Another way to fix this would be to change the initialization sequence, but this is another story. Closes #3749378

[devtools] even better: use a plain Connection (server-side) object for pre_setup The previous commit switched from passing a Session to a ClientConnection. This fixes unittest_ldapsource's pre_setup_database, which relied on a server-side object to get at the repo.