tests: add unittest.main() call

tests/test_piper_tts.py

@@ -7,6 +7,7 @@
 """Tests for PiperTTSService."""
 
 import asyncio
+import unittest
 
 import aiohttp
 import pytest
@@ -143,3 +144,7 @@ async def test_run_piper_tts_error(aiohttp_client):
         assert "status: 404" in up_frames[0].error, (
             "ErrorFrame should contain details about the 404"
         )
+
+
+if __name__ == "__main__":
+    unittest.main()