let post_link use original post title as title attribute

[ppwwyyxx]

What does it do?

This PR modifies the post_link tag so that the "title" attribute will contain the actual post title.

I think this new behavior is better:

• Same as the old behavior, the text will still show the title that users specified as arguments in post_link tag
• But once hovered, the tooltip will show full title of the actual post

In the old behavior:

• Once hovered, the tooltip will have the same content as the existing text, therefore the tooltip is not useful.

Screenshots

The screenshot shows the new behavior, where tooltip text is the actual post title

Pull request tasks

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

[github-actions]

How to test

git clone -b post_link https://github.com/ppwwyyxx/hexo.git
cd hexo
npm install
npm test

[coveralls]

Coverage remained the same at 98.031% when pulling a99693b on ppwwyyxx:post_link into afc4d7f on hexojs:master.

[ppwwyyxx]

The failures in github actions are:

Run marocchino/sticky-pull-request-comment@v2
Error: Request failed due to following response errors:
 - Resource not accessible by integration

this seems unrelaed to this PR

[ppwwyyxx]

Can maintainers take a look at this simple PR? thanks

[ppwwyyxx]

Can maintainers take a look? Thanks!