Attaching a behavior to `currentSite` differs between unit tests and web request

Description

I'm attaching some custom behaviors to the Site model. It seems like currentSite is created and set before I can add the the Site::EVENT_DEFINE_BEHAVIORS event listener. As a workaround, I'm setting the behavior directly on the currentSite in my module's init method.

public function init()
{
  parent::init();

  Craft::$app->sites->currentSite->attachBehavior('site-behavior', SiteBehavior::class);
}

This works fine on web requests, but when running unit tests, things seem to bootstrap in a different order.

Calling Craft::$app->sites->currentSite throws a SiteNotFoundException.

See: https://github.com/craftcms/cms/blob/develop/src/services/Sites.php#L515-L522

If I wrap the above code in a try/catch then my tests all run correctly. I can't test for the presence of currentSite or primarySite because they both throw an exception.

public function init()
{
  parent::init();

  try {
    Craft::$app->sites->currentSite->attachBehavior('site-behavior', SiteBehavior::class);
  } catch (Exception $e) {
    Event::on(
        Site::class,
        Site::EVENT_DEFINE_BEHAVIORS,
        function (DefineBehaviorsEvent $event) {
          $event->sender->attachBehavior('site-behavior', SiteBehavior::class);
        }
      );
  }
}

Steps to reproduce

1. Add a call to Craft::$app->sites->currentSite in the init method of a Craft Module.
2. Run unit tests to throw a SiteNotFoundException exception.

Additional info

• Craft version: 3.6.18
• PHP version: 7.4.24
• Database driver & version: MySQL 5.7.29
• Plugins & versions:

{
    "require": {
        "akeneo/api-php-client": "^6.0",
        "am-impact/amcommand": "3.1.4",
        "clubstudioltd/craft-asset-rev": "6.0.2",
        "craftcms/aws-s3": "1.2.11",
        "craftcms/cms": "3.6.18",
        "craftcms/commerce": "3.3.0.1",
        "craftcms/commerce-stripe": "2.3.2.1",
        "craftcms/feed-me": "4.3.5.1",
        "craftcms/redactor": "2.8.5",
        "enovatedesign/craft-style-inliner": "2.3",
        "fruitstudios/linkit": "1.1.12.1",
        "http-interop/http-factory-guzzle": "^1.0",
        "hubspot/api-client": "^2.8",
        "matt-west/craft-recaptcha": "1.5.2",
        "nystudio107/craft-retour": "3.1.53",
        "nystudio107/craft-seomatic": "3.4.9",
        "php-http/guzzle6-adapter": "^2.0",
        "spatie/craft-ray": "1.1.4",
        "studioespresso/craft-scout": "dev-master",
        "superbig/craft3-imgix": "2.1.0",
        "supercool/tablemaker": "2.0.1",
        "surprisehighway/craft-avatax": "2.1.6",
        "verbb/navigation": "1.4.14",
        "verbb/postie": "2.4.10",
        "verbb/super-table": "2.6.7",
        "viget/craft-classnames": "1.0.3",
        "viget/craft-viget-base": "3.1.2",
        "vlucas/phpdotenv": "^3.4.0",
        "yiisoft/yii2-redis": "^2.0"
    },
    "require-dev": {
        "codeception/codeception": "^4.1",
        "codeception/module-asserts": "^1.0.0",
        "codeception/module-phpbrowser": "^1.0.0",
        "codeception/module-yii2": "^1.1",
        "yiisoft/yii2-shell": "^2.0.3"
    },
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/vigetlabs/craft-scout.git"
        }
    ],
    "autoload": {
        "psr-4": {
            ... ⚠️ client code hidden
        }
    },
    "config": {
        "sort-packages": true,
        "optimize-autoloader": true
    },
    "scripts": {
        "post-root-package-install": [
            "@php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ]
    }
}

Start with src/services/Sites.php around lines 515-522 and the module init code described in the issue. Compare how currentSite is initialized during web requests and unit-test bootstrap, then trace when Site::EVENT_DEFINE_BEHAVIORS is dispatched. Done means custom Site behaviors can be attached consistently without catching SiteNotFoundException.