Update unit tests #3300

tests/composer.json

@@ -1,7 +1,7 @@
 {
     "name": "una.io/una-tests",
     "require-dev": {
-        "phpunit/phpunit": "6.*",
+        "phpunit/phpunit": "8.*",
         "sebastian/phpcpd": "*",
         "phpmd/phpmd": "@stable"
     }