chore(composer): upgrade test suite to Laravel 6

Author: kslimani

composer.json

@@ -8,8 +8,8 @@
         "laravel/passport": ">=4.0"
     },
     "require-dev": {
-        "orchestra/testbench": "^3.5",
-        "phpunit/phpunit": "^7.5"
+        "orchestra/testbench": "^4.0",
+        "phpunit/phpunit": "^8.3"
     },
     "autoload": {
         "psr-4": {