Refactor `lib/__tests__/plugins.test.js` (#5300)
Refactor `lib/__tests__/plugins.test.js` (#5300)

Reduce callbacks as much as possible via `async/await` syntax.
(see also <https://jestjs.io/docs/asynchronous>)
stylelint/stylelint,stylelint/stylelint,stylelint/stylelint,stylelint/stylelint