Create one SessionFactory per class in tests

[DavideD]

Not critical or urgent.

At the moment we open/close a session factory after each single test and therefore running the schema generation queries each time.

We could save some time creating the factory only once.
This requires:

• Add appropriate populateDb/cleanUpDb methods before and after each tests (if they are missing)
• Create the factory statically in the base class

Our build is already quite fast with some db but it might help with the slower ones.

[DavideD]

Create the factory statically in the base class

Actually, we might be able to use a JUnit rule to inject the factory

[gavinking]

If you do this, make sure that you recreate the factory after any test failure. Otherwise, the failure of one test method would have a cascading effect on all the other tests in that class (because its data won't be cleaned up).