[8.x] Fix forgetMailers with MailFake

[reindert-vetter]

I use \Illuminate\Support\Facades\Mail::forgetMailers() to force a new mailer to be used. But I can't use that because my test will fail. That's because that method doesn't exist in \Illuminate\Support\Testing\Fakes\MailFake. It makes sense that \Illuminate\Support\Testing\Fakes\MailFake also has that method.

Error:

Error : Call to undefined method \Illuminate\Support\Testing\Fakes\MailFake::forgetMailers()
 (...)/vendor/laravel/framework/src/Illuminate/Support/Facades/Facade.php:261

---

Workaround:

        $facadeRoot = Mail::getFacadeRoot();
        if (method_exists($facadeRoot, 'forgetMailers')) {
            Mail::forgetMailers();
        }