Add an example about testing

[jtpio]

Something that comes back often is the question of testing. So there could be an example about this that would cover several points such as:

• setting up jest
• reusing helpers from @jupyterlab/testutils
• spawning kernels and launching the tests with the run-test.py helper

There is already some documentation about this in:

https://jupyterlab.readthedocs.io/en/stable/developer/extension_dev.html#testing-your-extension

And we could draw some inspiration from the @jupyterlab/jupyterlab-git and @jupyterlab/debugger extensions.

[jtpio]

An example using galata would be really nice: https://github.com/jupyterlab/galata

[jtpio]

Hopefully this example should now be easier to make, since some testing tooling and infrastructure is now available in the cookiecutter: jupyterlab/extension-cookiecutter-ts#207