Add /test to .npmignore

In a new GTS setup when one drops a `.spec.ts` file into `/test` and `import`s anything from `mocha` or `chai`, this leads to an ESLint warning for [no-unpublished-import](https://github.com/mysticatea/eslint-plugin-node/blob/v11.1.0/docs/rules/no-unpublished-import.md).

As the 💡 in *Rule Details* suggests, a `.npmignore` holding one line for `/test` could lead to a less disruptive experience for new setups.