Merge pull request #247 from reactphp/3.x-update-test-environment-for-PHP-7.2

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

Author: clue

composer.json

@@ -32,7 +32,7 @@
         "react/promise": "^3.2 || ^2.7 || ^1.2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6 || ^7.5",
+        "phpunit/phpunit": "^9.6 || ^8.5 || ^7.5",
         "react/async": "^4.3 || ^3 || ^2",
         "react/promise-timer": "^1.11"
     },