[keycloak-dev] Moved user session model tests to testsuite/integration

I moved UserSession related tests to: https://github.com/stianst/keycloak/blob/master/testsuite/integration/src... The reason is that UserSession tests rely on Realms and Users so need a ModelProvider to run. It also makes it easier to run it with different configurations. I think we should move the rest of the tests in model/tests to the same place. This will help once we split the model into RealmProvider and UserProvider. If everyone is happy with this I can do it next week.