Disallow .only in mocha test files.

.eslintrc.json

{
  "env": {
    "mocha": true
  },
  "rules": {
    "max-nested-callbacks": ["error", 7],
    "no-restricted-properties": [
      "error",
      { "object": "describe", "property": "only" },
      { "object": "context", "property": "only" },
      { "object": "it", "property": "only" }
    ]
  }
}