cubicweb #847652 re enable testing agains real db [open]

This is precious at least when porting to a new rdbms (who said sqlserver?).

the generated skeletons need fixing and devtools probably needs some good hitting on the head.

reminder: this feature is supposed to work with:

cd mycube/test
pytest --source path/to/etc/cubicweb.d/myinstance/source

with --source coming from (generated by cwctl create) and also generated.

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