First off, welcome to nukeworker. Glad to see an intelligible first post
What you're saying sounds pretty typical of the hiring process, I wouldn't be too worried yet. If the positions aren't going to be filled until September it is a little surprising that they've contacted you this early, but there's no reason to be concerned about the listing being removed. That just means that think they've gotten enough qualified applicants already. As an example I just got a call yesterday to come in and take the POSS/interview for an AO position which I applied for in late February. The listing for that one has been offline since mid to late March and the class is supposed to be filled by July 1. So that gives you a rough estimate of how these timelines can work out. Thankfully this time I was able to tell them "Thank you but I've already accepted another position."
In your present situation I would suggest persistence. Email the recruiter about once a week just to "check in" on the process while displaying your continued interest. HR reps have a tendency to forget about you if you let them. If your recruiter hasn't given you test dates yet then ask for them. Don't be a pest, but do make sure to give them some good reasons to get you in for an interview. Good luck and keep us posted with regards to your future success.
-Tim