fix: run cargo test under xvfb-run; clean up unused imports and mut warning

ahfail_tests.rs calls on_activation which initialises GTK — needs a virtual
display in CI just like the Meson test step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -29,7 +29,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Run Rust tests
-        run: cargo test
+        run: xvfb-run cargo test
 
       - name: Meson build
         run: |