ci: fix unittest discover to use top-level-directory for package imports

The test_aiohttp_adapter module uses a relative import
(.aio.aiohttp_adapter_test). Without --top-level-directory ., unittest
discover imports test files as top-level modules rather than as members
of the pusher_tests package, breaking relative imports.

Author: git-wright

.github/workflows/test.yml

@@ -28,4 +28,4 @@ jobs:
         run: pip install -r requirements.txt
 
       - name: Run test suite
-        run: python -m unittest discover -s pusher_tests
+        run: python -m unittest discover -s pusher_tests --top-level-directory .