Shut down kernels after running the tests

[jtpio]

Fixes #231.

Two changes:

• Add a stop method to VoilaApp to shut down the kernels
• Call that method when in the tests before clearing the voila_app instance

It's probably simple enough for now to fix the issue, but we might want to reconsider this approach later on especially regarding pytest-tornado (as pointed out by @martinRenou in #231)

[martinRenou]

Mmmmh. Tests are failing on my side. But I would not trust my conda environment this time :P Better trust the CI

[jtpio]

Do the tests run in a new fresh environment, after running python -m pip install ".[test]" (without the -e) and installing the test_template?

[maartenbreddels]

LGTM

[maartenbreddels]

@martinRenou I'm assuming that is the case indeed :)