Automatically test the Greeter's Disable All Networking option

This is best done by:

Checking that NetworkManager is not running.
Using the ‘no network devices are present’ step from the (not yet merged as of writing this) test/6302-mac-spoofing branch.
Checking that there’s no network traffic for a while after booting Tails, just to be really sure, I guess.

Feature Branch: test/10340-disable-all-networking

Original created by @anonym on 10340 (Redmine)

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