Improve test coverage

Check List

Please check followings before submitting a new feature request.

• I have already read Docs page
• I have already searched existing issues

Feature Request

This repo's unit test coverage currently stands at .

For anyone who is interested in improving it, there are a few ways:

1. Search for "missing-unit-test" to locate functions that haven't been covered.
2. Click on the coverage badge link and it shows all the relevant lines.
3. Line coverage report is also shown in the CI job log.

hexo-util also needs help.