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)

js/server.js

@@ -72,11 +72,7 @@ function Server (config) {
 			app.use(helmet(config.httpHeaders));
 			app.use("/js", express.static(__dirname));
 
-			let directories = ["/config", "/css", "/fonts", "/modules", "/vendor", "/translations"];
-			if (process.env.JEST_WORKER_ID !== undefined) {
-				// add tests directories only when running tests
-				directories.push("/tests/configs", "/tests/mocks");
-			}
+			let directories = ["/config", "/css", "/fonts", "/modules", "/vendor", "/translations", "/tests/configs", "/tests/mocks"];
 			for (const directory of directories) {
 				app.use(directory, express.static(path.resolve(global.root_path + directory)));
 			}