First of all, you should use the WordPress modules for Codeception, which includes the PHPunit WPTestCase and many other tools.
There are two basic approaches to testing something like this in WordPress:
You can test it with a browser. This is called an acceptance test. You list all the actions that prove the concept you are testing, and you run a browser [or browser simulator] to do the task step by step. You can look into the database for the artifacts you expect to see as a result, and prove it happens correctly. In your case, you might want to setup some posts, click the multiple authors, and then check the database for the meta tags you expect. It might look something like:
$I = /*am a */ new AcceptanceTester($scenario);
$I->loginAsAdmin();
$I->amOnPage("/wp-admin/post-new.php");
$I->fillField('#content', "lorum ipsum");
$I->click('#publish');
The other approach is to encapsulate your action in an OOP class, and test it using the WPUnit test tools. Codeception uses the same PHPUnit library the WordPress core teams uses, plus a bunch of other tools. The WPUnit approach loads a database into memory, and can do things like setup dummy posts for your plugin to work on. So in your case, you might have a class:
class SystemActionSaveMetaTags{
public function doSaveMetaTags()
}
You might have a test called
itShouldSaveMetaTags(){
$id = wp_insert_post();
$SystemActionSaveMetaTags = new SystemActionSaveMetaTags;
$SystemActionSaveMetaTags->doSaveMetaTags($id);
$this->assertTrue($this->checkForCorrectMetaTags($id));
}
Here is a tutorial on the subject, as it relates to WordPress:
https://wp-bdd.com/wp-codeception-tutorial/