Add a front test framework + initial tests

This PR introduces some early tests on the CI and add some initial code to support it:

• npm run test
  • run frontend tests using jest
  • add some initial tests
• ̀npm run lint
  • fix code errors
  • ignore ext/ecodiag from the linter

This code is only a start, let's augment this every time we feel that adding tests is important :)