test: change client/test_client.py to client/test_client_factory.py in tests/README.md (#520)

"client/test_factory.py" no longer exists.

Author: sokoliva

tests/README.md

@@ -2,7 +2,7 @@
 
 1. Run the tests
     ```bash
-    uv run pytest -v -s client/test_client.py
+    uv run pytest -v -s client/test_client_factory.py
     ```
 
 In case of failures, you can cleanup the cache: