[tests] migrate from jest to vitest (#3940)

This is a big change, but I think it's a good move, as `vitest` is much
more modern than `jest`.

I'm excited about the UI watch feature (run `npm run test:ui`), for
example - it's really helpful and saves time when debugging tests. I had
to adjust a few tests because they had time related issues, but
basically we are now testing the same things - even a bit better and
less flaky (I hope).

What do you think?

tests/unit/modules/default/calendar/calendar_fetcher_utils_spec.js

@@ -1,7 +1,6 @@
 global.moment = require("moment-timezone");
 
 const ical = require("node-ical");
-const { expect } = require("playwright/test");
 const moment = require("moment-timezone");
 const CalendarFetcherUtils = require("../../../../../modules/default/calendar/calendarfetcherutils");
 

tests/unit/modules/default/utils_spec.js

@@ -12,7 +12,7 @@ describe("Default modules utils tests", () => {
 
 		beforeEach(() => {
 			fetchResponse = new Response();
-			global.fetch = jest.fn(() => Promise.resolve(fetchResponse));
+			global.fetch = vi.fn(() => Promise.resolve(fetchResponse));
 			fetchMock = global.fetch;
 		});