Refactor `lib/__tests__/ignore.test.js` (#5281)
Refactor `lib/__tests__/ignore.test.js` (#5281)

This change aims to improve the test code readability via async/await syntax or Jest's matchers.
(avoiding callbacks as much possible)

See also <https://jestjs.io/docs/asynchronous>
stylelint/stylelint,stylelint/stylelint,stylelint/stylelint,stylelint/stylelint