ci: fix test file references and hardcoded paths

- Remove non-existent test_mime_detection.py from unit tests
- Use relative paths in security tests for container compatibility

.gitea/workflows/ci.yml

@@ -117,7 +117,7 @@ jobs:
 
       - name: Run unit tests
         run: |
-          pytest tests/test_api.py tests/test_database.py tests/test_mime_detection.py \
+          pytest tests/test_api.py tests/test_database.py \
                  tests/test_paste_*.py tests/test_metrics.py tests/test_pki.py \
                  -v --tb=short