Add NixOS test for Gnome Flashback

[worldofpeace]

Describe the bug
Admittedly, I don't believe I ever tested the session when working on #68608.
Then I realized our base testing lacks any coverage for this particular session, and we should be testing this.

[worldofpeace]

cc @hedning @jtojnar

[stale]

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This has been automatically marked as stale because it has had no activity for 180 days.
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