As said before, if you've got a GE ticket, entry for the testweekend is included.
If you want to get close to the cars and into the pitlane, the testweekend is the place to be. Pitlane is open on Saturday all day to stroll around and watch the cars going into scrutineering, pitboxes ... Several teams will do pitstop practice, lots of drivers to spot in & around the paddock ... you'll get access that you won't get during raceweek. Guess it will be the same on Friday (scrutineering even started on Thursday this year with 60 entries).
Don't know about campsites, didn't see any campers around the track this year on BN, Rouge or BSJ. Hotels are not too expensive during the testweekend, if you book early you might even find a few bargains.