Automatically test that udev-watchdog is monitoring the right device when started from USB

#5560 (closed) didn’t make it clear that this should be tested as well, although our manual test suite reads:

- `udev-watchdog` must monitor the right device when booted off USB (automate: [[!tails_ticket 5560]])
- `udev-watchdog` must monitor the right device when booted off DVD (automate: [[!tails_ticket 5560]])

So here’s a ticket to add the missing test.

Feature Branch: kytv:test/9890-udev-watchdog-from-usb

Related issues

Related to #5560 (closed)

Original created by @intrigeri on 9890 (Redmine)

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