fix electron tests mocking dates (#3599)

fixes #3597 

Changes:
- electron tests: add mocking to `electron.js` for mocking the server
side, before only the browser side was mocked
- publish "/tests/configs" and "/tests/mocks" always in `server.js`,
this reverts a change done with latest release, we need this for
debugging (otherwise you get on the screen that your config has errors
but config check is successful ...)
- revert hotfix in
`tests/configs/modules/calendar/show-duplicates-in-calendar.js`
- fix `tests/configs/modules/calendar/custom.js` to allow events in the
past (~~I don't know how this could work before~~ when testing css
classes `yesterday` and `dayBeforeYesterday` --> it worked before
because the server side did not mock and therefore was not in the past)

tests/configs/modules/calendar/custom.js

@@ -13,6 +13,8 @@ let config = {
 				calendars: [
 					{
 						maximumEntries: 5,
+						pastDaysCount: 5,
+						broadcastPastEvents: true,
 						maximumNumberOfDays: 10000,
 						symbol: "birthday-cake",
 						fullDaySymbol: "calendar-day",