Replace Buster.js with Karma+Mocha

[jodal]

The main advantage of the Buster.js test framework is that the same test suite can be executed both on Node.js and in the browser. I believe the same is achievable with the Karma test runner together with the Mocha test framework.

Since Buster.js is barely maintained, Mopidy.js' test suite should be ported to Karma+Mocha.

When doing so, we can either keep referee for assertions through the use of karma-referee or rewrite them to use the Chai assertion library.

[jcass77]

Mopidy Musicbox Webclient has started to use Karma+Mocha+Chai, and may contain a few useful configuration and test script examples to get this started.

[jodal]

Mopidy.js' test suite was rewritten from Buster.js to Jest in commit fc1bd39.