End to End testing

[jtpio]

Problem

Visual regressions can sometimes be introduced when making changes to Voila.

For example #846 is going to drastically change the way the frontend works, and it would be great to check if this big change is going to affect some of the Voila logic, rendering or features.

Proposed Solution

We should add Visual Regression tests, which has already been explored in #574 and #821.

One option would be to follow what JupyterLab, ipywidgets and bqplot (and potentially other repos) have done and adop Galata: https://github.com/jupyterlab/galata

If testing the Voila rendered page directly is not feasible with Galata, we might able to use the JupyterLab preview extension instead.

Additional context

• test: add render test with pyppeteer - chromium based #574
• Playwright end to end test for the preview extension #821
• https://github.com/jupyterlab/jupyterlab/tree/master/ui-tests
• https://github.com/jupyter-widgets/ipywidgets/tree/master/ui-tests