The way the PHPUnit tests are currently being scaffolded does not work with PHP 8.0+:

PHP Parse error:  syntax error, unexpected token "match", expecting identifier in
phar://phpunit/Framework/MockObject/Builder/Match.php on line 15

To fix this, the tests should be scaffolded in such a way that they make use of the WordPress Core compatibility code found in https://core.trac.wordpress.org/changeset/49037. However, that will require a rewrite to make the scaffolded tests rely on PHPUnit being executed as a Composer dev dependency.