Update Mocha.js

We're using an ancient version of Mocha.js: 4.1.0.

This involves:

- [ ] Migrating Mocha's from `opts` and to the now canonical [configuration file](https://mochajs.org/#configuring-mocha-nodejs); the way that the configuration options are set for the current Mocha.js setup is [outdated][1] and [obscure][2].

[1]: https://github.com/mochajs/mocha/blob/34643e4c0821aeb8d6977c1942bc106c9363789a/docs/index.md#--opts-path
[2]: https://github.com/mochajs/mocha/blob/v4.0.1/bin/options.js#L21

Start by locating Karma's current Mocha.js opts setup and compare it with Mocha's canonical configuration-file documentation linked in the issue. Migrate the existing options without changing test behavior, then run the repository's test suite to confirm the updated Mocha.js setup works.

Written by the indexing model from the issue text.