Test MAC spoofing on Wheezy for hot-plugged network interfaces

Once feature/mac-spoof is merged into feature/wheezy, we should be unaffected by #6294 (closed), at least for network interfaces that are present at boot time. But we should verify we’re also safe for hot-plugged ones.

Parent Task: #6294 (closed)

Original created by @intrigeri on 6890 (Redmine)