Resolve "Fix Pipeline"

**What did I do?**

* change `.gitlab-ci.yml` to basically reproduce docker compose up and npm run jest
  1. Go into server and run `npm install`
  1. Go into client and run `npm install`
  1. Go into server and run `npm run build`
  1. Go into server, start the server and run all tests in `server/tests`
  1. Go into server, start the server, go into client, start the client and run all tests in `client/tests`
* Create Testing documentation under `doc/testing.md`
* install `jest` and `ts-jest` and all their dependencies as dev-dependencies
* Create config files for jest under `client/jest.config.js` and `server/jest.config.js`
* Create basic tests that also serve as examples under `server/tests/connection.test.ts` and `client/tests/connection.test.ts`
  * The server tests basically try some GET requests to some API Endpoints defined in `server/src/routes.ts` and expect 404 errors
  * The client just polls the index.html file of the frontend
* change the `tsconfig.json` files to lint(?) the test directories