Thanks for working on this!

I've tested using the testing instructions:

Case 1: Default

• Verify the font is uploaded to /wp-content/fonts and the font url is set to site_url/wp-content/fonts/your-font.ttf ✅

Case 2: Custom

• Changed the fonts directory by adding the following snippet to my active theme, and verified the font is uploaded to /custom-fonts-dir ✅

function filter_fonts_dir( $font_settings ) {
    $new_sub_dir = '/custom-fonts-dir';

    $font_settings['dir'] = rtrim( $font_settings['dir'], '/' ) . $new_sub_dir;
    $font_settings['url'] = rtrim( $font_settings['url'], '/' ) . $new_sub_dir;

    return $font_settings;
}
add_filter( 'fonts_dir', 'filter_fonts_dir' );

Case 3: Multi-site

• Verify the font is uploaded to /wp-content/fonts/your-site-id and the font url is set to site_url/wp-content/fonts/your-site-id/your-font.ttf ✅

This only worked if I switched to a site that wasn't the primary site, but I think this is the correct logic.

This all looks great to me. I've left some comments for tiny suggestions but otherwise I think this is ready to bring in 🎉

I added a few commits to make the fonts_dir filter return an array shape equal to the upload_dir filter. Why? because we are using the values of fonts_dir to update upload_dir which will be used to handle the fonts uploaded.

Apart from that, I think it simplifies the code a little.

I removed the no longer needed tests and added new ones to try the filter and its removal.

Tests made:
☑️ set filter works
☑️ unset filter works
☑️ upload fonts from multisite
☑️ upload fonts from regular site
☑️ tests pass

LGTM