test: improve coverage

[SukkaW]

What does it do?

#3984

Continuation of #4050

How to test

git clone -b test-coverage-1 https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[SukkaW]

@stevenjoezhang GitHub downs again and causes all tests are hanged.

I will merge the PR after GitHub's services restored and tests are finished.

[stevenjoezhang]

It is up to you to decide~
I tested it locally and the new test cases work as expected

[SukkaW]

I tested it locally and the new test cases work as expected

Well, I need to see coverage change from coverage bot's report.

[SukkaW]

It appears that the GitHub's services is now restored and the PR is now able to track the changes.

[coveralls]

Coverage increased (+0.2%) to 97.993% when pulling 2d81d7d on SukkaW:test-coverage-1 into 89b2a9a on hexojs:master.