Write unit tests and publish coverage

[jschaeff]

Make github action to run tests and test coverage, generate a badge and put it in the README.

[vpet98]

Done. A new push by github actions adds the badge in the webservice README.

I will write some more tests trying to increase the percentage. I think some parts of the code would really need a real database to be tested, so the percentage has an upper limit, which definitely is below 100%.