chore: make mocha rc configs dotfiles (badges#3065)

Fixes badges#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

package.json

@@ -82,7 +82,7 @@
     "test:js:server": "echo \"Deprecated; run `npm run test:core` instead.\" && npm run test:core",
     "test:js:frontend": "echo \"Deprecated; run `npm run test:frontend` instead.\" && npm run test:frontend",
     "test:js:package": "echo \"Deprecated; run `npm run test:package` instead.\" && npm run test:package",
-    "test:frontend": "cross-env NODE_ENV=test mocha --config mocharc-frontend.yml \"frontend/**/*.spec.js\"",
+    "test:frontend": "cross-env NODE_ENV=test mocha --config .mocharc-frontend.yml \"frontend/**/*.spec.js\"",
     "test:core": "cross-env NODE_CONFIG_ENV=test mocha \"core/**/*.spec.js\" \"lib/**/*.spec.js\" \"services/**/*.spec.js\"",
     "test:package": "mocha \"gh-badges/**/*.spec.js\"",
     "test:entrypoint": "cross-env NODE_CONFIG_ENV=test mocha entrypoint.spec.js",