Use PHPUnit v8

Author: stancl

composer.json

@@ -24,7 +24,7 @@
         "illuminate/database": "^6.0|^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.0",
+        "phpunit/phpunit": "^8.0",
         "orchestra/testbench": "^5.1"
     }
 }