Tests: fix the AuthSession "test from another process"

This test is failing with Qt 5.6, getting stuck when calling QDBusConnection::connectTo() from the child process. Given that calling fork() from a test function is generally not a good idea, we refactor this test to avoid calling fork() directly, and use a separate QProcess instead.

While doing this, we move the refactored test to the signond-tests directory, since this test is not testing the client library at all, but only signond's functionality.