Mobile app integration test CI

We should have a way to test ObjectBox runs on mobile (simulator) Android and iOS. From the quick look in the past I think our current tests (/test folder) can't be just executed on the target platform and we would rather need to have an app that instead runs the test suite itself. I've done this quickly in the past with the /lib/integration_test.dart. If this is the right approach, it should be moved to a separate dart/flutter project and run from CI for both mobile platforms (when iOS support #47 is merged)