Update dependency react-parallel/pool-tests to v6

Author: reactphp-parallel-renovate-runner[bot]

composer.json

@@ -24,7 +24,7 @@
     "wyrihaximus/ticking-promise": "^3.1"
   },
   "require-dev": {
-    "react-parallel/pool-tests": "^5.1.0",
+    "react-parallel/pool-tests": "^5.1.0 || ^6.0.0",
     "wyrihaximus/async-test-utilities": "^10.5.0",
     "wyrihaximus/makefiles": "^0.7.15"
   },