Refactor `lib/__tests__/integration.test.js` (#5284)
Refactor `lib/__tests__/integration.test.js` (#5284)

* Refactor `lib/__tests__/integration.test.js`

- Reduce callbacks as much as possible via `async/await` syntax. (see also <https://jestjs.io/docs/asynchronous>)
- Unify indentation of tested CSS-like code for readability.
- Add a utility function to reduce duplication.

* Re-indent template literals via `stripIndent()`
stylelint/stylelint,stylelint/stylelint,stylelint/stylelint,stylelint/stylelint