Add test suite

[WateryStoneTree]

With all the features supported now it is probably a good idea to have some kind of automated testing.

Adding tests could also be a easy way to contribute for people who don't know too much about the Svalte/JS/App etc ecosystem.

Things to test:

• Import of decks and valid parsing
• Layout on phone/web/pc
• Check if quiz section can be finished.
• Check correct application of options and settings on decks and app behavior (e.g. changing the number of new cards per day, does it apply to all decks correctly etc.)
• Account related features.
• Usage of offline audio files
• usage of different dictionaries