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/electron/helpers/global-setup.js

@@ -8,6 +8,10 @@ exports.startApplication = async (configFilename, systemDate = null, electronPar
 	global.page = null;
 	process.env.MM_CONFIG_FILE = configFilename;
 	process.env.TZ = timezone;
+	if (systemDate) {
+		process.env.MOCK_DATE = systemDate;
+	}
+
 	global.electronApp = await electron.launch({ args: electronParams });
 
 	await global.electronApp.firstWindow();
@@ -34,6 +38,7 @@ exports.stopApplication = async () => {
 	}
 	global.electronApp = null;
 	global.page = null;
+	process.env.MOCK_DATE = undefined;
 };
 
 exports.getElement = async (selector) => {