Use same version of phpunit as GH ubuntu-latest image

https://github.com/actions/runner-images/blob/521d3890c483f4fbfa841f36728c19fd55ade129/images/ubuntu/Ubuntu2204-Readme.md#php-tools

composer.json

@@ -16,7 +16,8 @@
 		"wp-coding-standards/wpcs": "^3.1",
 		"phpcompatibility/phpcompatibility-wp": "*",
 		"php-parallel-lint/php-parallel-lint": "^1.3",
-		"yoast/phpunit-polyfills": "2.0.0"
+		"yoast/phpunit-polyfills": "2.0.0",
+		"phpunit/phpunit": "8.5.38"
 	},
 	"config": {
 		"allow-plugins": {
@@ -30,6 +31,9 @@
 		],
 		"fix-cs": [
 			"@php ./vendor/bin/phpcbf"
+		],
+		"test": [
+			"phpunit"
 		]
 	}
 }