Take the output of https://gist.github.com/mfn/256636242cbe8a51252ce28181a6b074 and rewrite an existing phpunit.xml to use only these test classes (mapped to files)

phpunit_xml_class_to_file.php

#!/usr/bin/env php
<?php declare(strict_types = 1);

$command = array_shift($argv);

if (3 !== count($argv)) {
    echo <<<USAGE
Usage: $command path/to/composer.json path/to/phpunit.xml path/to/slice.xml > new_phpunit.xml

Based on your composer PSR-4 autoload configuration, the classes referenced
in slice.xml will be converted to test files and replace any <testsuites> in
your phpunit.xml with a list of all the files.

USAGE;

    exit(1);
}

$composerJson = json_decode(file_get_contents($argv[0]), true, 512, JSON_THROW_ON_ERROR);
$psr4Autoloaders = array_merge(
    $composerJson['autoload-dev']['psr-4'] ?? [],
    $composerJson['autoload']['psr-4'] ?? [],
);

if (!$psr4Autoloaders) {
    throw new RuntimeException('No PSR-4 autoloaders found in composer.json!');
}

$mapper = new MapClassToFile($psr4Autoloaders);

$classes = [];

$slicedXml = simplexml_load_file($argv[2]);

foreach ($slicedXml->testCaseClass as $class) {
    $classes[$class['name']->__toString()] = true;
}
unset($slicedXml);

$phpunitXml = simplexml_load_file($argv[1]);

// Remove all existing suites
unset($phpunitXml->testsuites->testsuite);

$testsuite = $phpunitXml->testsuites->addChild('testsuite');
$testsuite->addAttribute('name', $argv[2]);

foreach (array_keys($classes) as $class) {
    $file = $mapper->map($class);
    $testsuite->addChild('file', $file);
}

echo $phpunitXml->asXML();

class MapClassToFile
{
    private array $psr4Autoloaders;

    public function __construct(array $psr4Autoloaders)
    {
        $this->psr4Autoloaders = $psr4Autoloaders;
    }

    public function map(string $class): string
    {
        foreach ($this->psr4Autoloaders as $namespacePrefix => $directory) {
            if ($this->startsWith($class, $namespacePrefix)) {
                $file = substr($class, strlen($namespacePrefix));
                $file = str_replace('\\', DIRECTORY_SEPARATOR, $file);

                return "$directory$file.php";
            }
        }

        $msg = "Did not find a matching namespace for class $class in PSR-4 autoloaders found in composer.json: ";
        $msg .= json_encode($this->psr4Autoloaders);

        throw new RuntimeException($msg);
    }

    /**
     * Starts the $haystack string with the prefix $needle?
     */
    private function startsWith(string $haystack, string $needle): bool
    {
        return 0 === strncmp($haystack, $needle, strlen($needle));
    }
}