update phpunit to be php8 compatible

Author: smoench

composer.json

@@ -68,7 +68,7 @@
     "require-dev": {
         "bamarni/composer-bin-plugin": "^1.3",
         "mikey179/vfsstream": "^1.6",
-        "phpunit/phpunit": "^8.1",
+        "phpunit/phpunit": "^8.5.12",
         "symfony/phpunit-bridge": "^4.2 || ^5.0"
     },
     "suggest": {