Re-enable and adjust test for desktop icons
f211f51b disabled a check due to #16283 (closed), which has been fixed since then. Let’s re-enable this test. But it relies on the desktop icons to be displayed by Nautilus, which is not the case anymore, so the test needs to be updated.
Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/16
Parent Task: #16281 (closed)