We still support rehandshakes, but the public API docs say otherwise
glib@bbbaae9f says we no longer support rehandshakes since GLib 2.64, but in fact we do. The code was never removed.
I have notes from half a year ago that say rehandshakes need to remain supported. Without remembering exactly why, I guess we don't necessarily need to allow triggering rehandshake via API request, but should still support rehandshakes if requested by the peer. So probably we should update our behavior to match the documentation... but that means all the rehandshake tests would have to be removed, since we can't request rehandshake without that API.