@kubaszymanowski the easiest thing to do, is to leave the tests in your composer package, then import the package into a test Laravel app and then use them from this apps tests using a PHP trait.
Here's an example I put online to test somebody else's package (The Garbage Man package)
https://github.com/ifpingram/laravel-garbage-man-host
https://github.com/ifpingram/laravel-garbage-man-host/blob/master/composer.json
https://github.com/ifpingram/laravel-garbage-man-host/blob/master/tests/PurgeCommandTest.php
HTH!