Add PHPUnit dependency for testing framework

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Co-Authored-By: Claude <[email protected]>

Author: JanTvrdik

composer.json

@@ -20,6 +20,7 @@
         "phpstan/phpstan-deprecation-rules": "^2.0.1",
         "phpstan/phpstan-phpunit": "^2.0.4",
         "phpstan/phpstan-strict-rules": "^2.0.3",
+        "phpunit/phpunit": "^9.6.22",
         "shipmonk/composer-dependency-analyser": "^1.8.1",
         "shipmonk/dead-code-detector": "^0.9.0",
         "shipmonk/name-collision-detector": "^2.1.1",