chore(backend): add test dependencies to requirements

添加 pytest、pytest-cov 和 pytest-asyncio 作为测试依赖项。

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/requirements.txt

@@ -20,3 +20,8 @@ accelerate
 librosa
 psutil
 torchaudio
+
+# Test dependencies
+pytest
+pytest-cov
+pytest-asyncio