tests: Add automated end-to-end scheduled tests

[maxpatiiuk]

Given that this extension relies on scraping the Google Chrome's web page quite a bit, the changes to layout on their side could break the extension.

It is bad when I am notified though this only though a negative review in the chrome web store.
Fortunately, I use the extension very often (or at least a subset of it's features), so would be notified of breakages relatively quickly. Still it would be nice to have some testing automated.

While it is a bit challenging to automate a test for a third-party website requiring authentication, I can try the following:

1. Have a github action run at a set interval
2. It will open Google Calendar in a headless browser and do the authentication using a test google account (before that, read if this is against Google's terms of service)
3. Then it would scrape the needed web content and compare it to saved data
4. It would also take a screenshot of the page, and compare it to saved screenshot to check if the "Condensed page styles" started to look differently

For the future:

• Do fewer customization options, and certainly fewer features that even I myself don't find useful (i.e Goals, in their present form)
• For the rest of the features, either use them regularly myself, or setup automated tests