Bump testing tools to the same version as server

Signed-off-by: Joas Schilling <coding@schilljs.com>

tests/integration/composer.json

@@ -1,8 +1,8 @@
 {
   "require-dev": {
-    "phpunit/phpunit": "~5.7",
-    "behat/behat": "^3.0",
-    "guzzlehttp/guzzle": "~5.0",
+    "phpunit/phpunit": "~8.4",
+    "behat/behat": "~3.5.0",
+    "guzzlehttp/guzzle": "6.3.3",
     "jarnaiz/behat-junit-formatter": "^1.3"
   }
 }