WIP: sib-core tests

Libraries

We use open wc, karma and chai.
The sample test is /tests/storeMixin.test.js.
Documentation: https://open-wc.org/testing/

Launch tests

1. npm install
2. npm run build:dev
3. npm run test or npm run test:watch

NB: It fails sometimes, depending on how long it takes to the store mixin to fetch the datas. It will be fixed in a future version

@sylvain @clement @balessan what do you think about this?