I don't think so, it looks like that might have to be done entirely via the programming API, and you need an application registered to do so (or the access token of another application) because it only works on one application: http://developers.facebook.com/docs/test_users/

You can definitely create a user and then turn him into a test account manually, I've created many test accounts that way. You have to visit a special URL that turns the account into a test account (can't remember it offhand, and I'm not going to post it anyway since if anyone clicks on it then clicks "accept" without looking, it nukes their regular account).

Can the test user then interact with the site in a mostly-regular way? I've never tried, clearly :)

Yes, except for the fact that it can't interact with regular users.