Tests: improved tests and test assets

Details:
- ConferenceServiceTest now uses the test data from Assets
- Assets now provides accounts with passwords
- Each test leaves the db populated after tests and purges the db before each individual test. This means that after running the tests, you will still have some test data in the db if you want some for running the application.
- Test accounts: alice@foo.com (has all permissions), bob@bar.com, eve@evil.com. Password is allways 'testtest'.