Add phpunit dependency to use fixed version in automated tests

Author: AlexVanderbist

composer.json

@@ -31,6 +31,7 @@
         "mockery/mockery": "^1.4",
         "orchestra/testbench": "^7.0|^8.0",
         "pestphp/pest": "^1.20",
+        "phpunit/phpunit": "^9.6",
         "spatie/invade": "^2.0",
         "spatie/laravel-ray": "^1.28"
     },