Recommend pytest --pyargs jupyterlab_server (#203)

Author: krassowski

CONTRIBUTING.md

@@ -16,5 +16,5 @@ To create a local test setup run the following commands (inside your virtual env
 git clone https://github.com/jupyterlab/jupyterlab_server.git
 cd jupyterlab_server
 pip install -e .[test]  # install test dependencies
-pytest
+pytest --pyargs jupyterlab_server
 ```