alda, 
wp-tests-strapon has been finally published on Packagist as
aldavigdis/wp-tests-strapon.PHPUnit 9 (which large parts of the WP community is stuck with) is now supported out of the box as well as the more recent version 10.
This means that it can be installed using
composer require --dev aldavigdis/wp-tests-strapondirectly.More at https://packagist.org/packages/aldavigdis/wp-tests-strapon
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