Update dependency phpunit/phpunit to v8.5.15

Author: renovate-bot

composer.json

@@ -8,7 +8,7 @@
     "php": ">=5.6|>=7.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "8.3.5",
+    "phpunit/phpunit": "8.5.15",
     "woocommerce/woocommerce-sniffs": "0.0.9"
   },
   "scripts": {