Switch to JupyterServer from @jupyterlab/testutils

[jtpio]

Fixes #445.

Changes

• Switch to JupyterServer from @jupyterlab/testutils, so there is no need for the run-test.py anymore and the deprecated JestApp (and tests become pure jest tests)
• Match the folder and test structure in core lab more closely

[jtpio]

The jest reporter now looks the same as in core lab too (since it's reusing the same settings):

[jtpio]

It looks like kernel specs (in particular the xpython spec) don't have to be installed manually anymore, most likely because the environment patching doesn't override the existing jupyter paths.

[jtpio]

For the record, this is the output of running jupyter --paths (before running the tests) on the GitHub Actions runner from the conda workflow:

config:
    /home/runner/.jupyter
    /usr/share/miniconda/envs/jupyterlab-debugger/etc/jupyter
    /usr/local/etc/jupyter
    /etc/jupyter
data:
    /home/runner/.local/share/jupyter
    /usr/share/miniconda/envs/jupyterlab-debugger/share/jupyter
    /usr/local/share/jupyter
    /usr/share/jupyter
runtime:
    /home/runner/.local/share/jupyter/runtime

[jtpio]

We should be able to add the wheel test back on CI after this: #431

[afshin]

Sweet!

👍