chore: setup PHPUnit for project tooling

.phive/phars.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <phive xmlns="https://phar.io/phive">
   <phar name="phpstan" version="1.11.1" installed="1.11.1" location="./tools/phpstan" copy="false"/>
+  <phar name="phpunit" version="^10.5.20" installed="10.5.20" location="./tools/phpunit" copy="false"/>
 </phive>

phpunit.dist.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit bootstrap="tests/phpunit-bootstrap.php">
+    <testsuites>
+        <testsuite name="Tests">
+            <directory>tests/</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>

tests/phpunit-bootstrap.php

+<?php
+
+if (!defined('QUIQQER_SYSTEM')) {
+    define('QUIQQER_SYSTEM', true);
+}
+
+if (!defined('QUIQQER_AJAX')) {
+    define('QUIQQER_AJAX', true);
+}
+
+require_once __DIR__ . '/../../../../bootstrap.php';