Ensure MAC Spoofing Panic message are correctly displayed (refs: #17779)

Udev may close child processes when a process associated with a rule (/etc/udev/rules) terminates. As mac spoofing notifications processes are launched in background, with a static wait, they may be killed before the message is effectively displayed. This patch makes tails-spoof-mac wait for those processes before exiting. It may makes tails-spoof-mac wait indefinitely. However, udev will ensure it terminates after 120 seconds. Additionally it reactivates some tests about this scenarios that were previously removed in 87663631.

Closes #17779 (closed).