[3.x] Update test environment for PHP 7.2 to compatible PHPUnit version

This changes ensures we can continue run PHPUnit on PHP 7.2 by updating PHPUnit to 8.5. This is due to recent improvements in composer as discussed in reactphp/event-loop#284.

Author: WyriHaximus

composer.json

@@ -34,7 +34,7 @@
         "react/stream": "^1.4"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6 || ^7.5",
+        "phpunit/phpunit": "^9.6 || ^8.5 || ^7.5",
         "react/async": "^4.3 || ^3",
         "react/promise-stream": "^1.4",
         "react/promise-timer": "^1.11"