Cannot run all tests with a single pytest command

[BoPeng]

pytest

fails because the test suite tries to new_window to create a new notebook, which does not work for JupyterLab.

Currently I have switched to running tests one by one in .travis.yml, namely

    - cd test && pytest test_frontend.py -v && pytest test_magics.py -v && pytest test_workflow.py -v

but we should fix new_notebook when we get a chance.