Can not run two tests with dictschema
Since e5fdb866f509, it is not possible to run two tests (e.g. a TrytonTestCase and a scenario) that involves module with DictSchema.
The problem is that a Cache instance with the same name is still registered in the BaseCache._instances when the second database setup the DictSchema class.
So for me, the design to instantiate a Cache in the __setup__ was wrong. And we must use a single Cache instance for all derivative classes.