chore: make mocha rc configs dotfiles (#3065)

Fixes #3054 

Apparently mocha 6's new auto-config discovery only works with dotfiles. This renames our mocha rc files accordingly: `mocharc.yml` --> `.mocharc.yml` and solely to be consistent `mocharc-frontend.yml` --> ` .mocharc-frontend.yml`

- https://github.com/mochajs/mocha/blob/master/lib/cli/config.js#L23
- https://mochajs.org/#configuring-mocha-nodejs
- https://github.com/mochajs/mocha/blob/master/CHANGELOG.md#tada-enhancements-1