Enhancements/add cypress

Checklist

[x] I documented the TypeScript code using JSDoc style.
[x] I added multiple screenshots/screencasts of my UI changes
[x] I translated all the newly inserted strings into German and English

Motivation and Context

We want to increase automatic test coverage for Apollon. We want to create e2e tests in Apollon, to have greater confidence about the quality of Apollon and lower the risk of introducing bugs. This PR sets up cypress for Apollon.

Description

I configured cypress and wrote a first small test with commands and a plugin to show that the setup works. I also had to move all unit testing files into a new directory /tests/unit and adjust all paths.

The test folder structure for our project is now:

src
    -- main
    -- stories
    -- tests
          -- unit  
          -- e2e

This PR provides the basis for further test coverage improvements. We should also going forward create e2e tests for features/bugfixes we implement.

Steps for Testing

In order to test this, one would have to run unit tests to ensure they are still working and run the newly created cypress test. We do that by:

yarn test && yarn test:e2e

ls1intum / Apollon