Use composer to require package with tests included

I want to reuse test code (e.g. mock classes) from a package. But i don't know how to tell composer to fetch the dependency with tests included.

My composer.json:

"require": {
   "some/package": "2.0.0"
}

composer.json of the other package (which has /src and /tests subfolders):

"autoload": {
    "psr-4": {
      "Some\\Namespace\\": "src/"
    }
},
"autoload-dev": {
    "psr-4": {
      "Some\\Namespace\\Tests\\": "tests/"
    }
},

This gives me only the /src folder under /vendor/some/package/.

I tried specifiying some/package: 2.0.0@dev without any effect.

Is this even possible with composer (and packagist)?