fix electron tests mocking dates

[khassel]

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)

[sdetweil]

looks good, so net have the electron process set the date for the test run, helper just passes it on thru env. cool