Upgrade PHPUnit to v7

Author: aboks

composer.json

@@ -21,6 +21,6 @@
         "slevomat/coding-standard": "^5.0 || ^6.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6"
+        "phpunit/phpunit": "^7"
     }
 }