The link tag and other tags does not with a filter chain.
So this is invalid
{% link test_sub.md | absolute_url %})
This is what I do for all my Jekyll 3 sites
[Link to a page]({{ site.baseurl }}{% link test_sub.md %})
In Jekyll 4, you can simply do the link tag and it will add the relative URL subpath for you.
[Link to a page]({% link test_sub.md %})
Most of the time you should not want to use absolute URLs. Use relative URLs. You will need an absolute URL for meta HTML tags, RSS feed maybe, sitemap…
I guess you could use url from config if it was set with a domain. To make this absolute.
[Link to a page]({{ site.url }}{{ site.baseurl }}{% link test_sub.md %})