Use global 'server' variable in test suite, stop server intervals in 'afterEach'

As talked about (https://github.com/rmehner/spotify-remote/pull/4#commitcomment-2568988), I changed the test suite to use a global `server` variable, which gets overridden in each tests, but the tests don't have to care about calling `server.stopIntervals()` any more, since that's done in `afterEach`.