fix: also search for coauthor posts by term slug

All Submissions:

[x] Have you followed the Newspack Contributing guideline?
[x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
[x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

When searching for posts from an author that is a WP User (not a guest author), we should also (actually maybe only) search by term slug.

This PR adds a few extra conditions to make sure we find posts by an author when searching by terms.

The specific case we are fixing here is from users that ended up with their Display Name as the term name, and thus the search term name using the user_login as input would never find anything. We also need to search by user_nicename and make sure we search in the slug field.

How to test the changes in this Pull Request:

Activate CoAuthors Plus
Create a user and assign a few posts to it
Create a post and assign this user as a secondary co-author (a co-author, but not the author on the post_author field in the database
On a page, add a Home Page posts block and filter by this author
Make sure all the posts show up, including the one they are not the main author (on trunk it won't show up)
Add this somewhere:

// Disable guest authors before CoAuthors plus is initialized
add_action(
    'init',
    function() {
        add_filter( 'coauthors_guest_authors_enabled', '__return_false' );
    },
    9
);

Make sure the block still fetch the correct posts.
Remove the snippet to re-enable Guest Authors
Create a guest author
Create a post and assign only the guest author to it.
You can confirm and see that the post_author field in the DB still points to your admin
In the HPP block, filter posts by the guest author and confirm it works
Now filter posts by your admin, and confirm the post by the guest author is NOT returned

Other information:

[x] Have you added an explanation of what your changes do and why you'd like us to include them?
[x] Have you written new tests for your changes, as applicable?
[x] Have you successfully ran tests with your changes locally?

Automattic / newspack-blocks