Merge pull request #275 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

@@ -29,7 +29,7 @@
     },
     "require-dev": {
         "phpstan/phpstan": "1.12.28 || 1.4.10",
-        "phpunit/phpunit": "^9.6 || ^7.5"
+        "phpunit/phpunit": "^9.6 || ^8.5 || ^7.5"
     },
     "autoload": {
         "psr-4": {