Test on lowest and highest dependencies

Switch to phpunit 8

Author: ihor-sviziev

composer.json

@@ -13,7 +13,7 @@
         "webonyx/graphql-php": "^14.9"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.0||^9.0"
+        "phpunit/phpunit": "^7.0||^8.0"
     },
     "autoload": {
         "classmap": [