cubicweb #1373384 Cached database setup for test [resolved]

When database setup in test are slow are frequently used. We would like to be able to cache it.

The basic idea is:

1. Provide a (<database setup function>, <identifier>) couple
2. Mark some test/testclass as using /pre_setup/ <identifier>
3. Run the setup function once. Cache the resulting database.
4. Restore the Cached database for each test marked as using /pre_setup/ <identifier>