Composer: update test dependencies

... to take advantage of the recently released new version of WP Test Utils.

Ref: https://github.com/Yoast/wp-test-utils/releases/tag/1.2.1

Author: jrfnl

composer.json

@@ -28,7 +28,7 @@
 		"composer/installers": "^1.12.0 || ^2.0"
 	},
 	"require-dev": {
-		"yoast/wp-test-utils": "^1.2.0",
+		"yoast/wp-test-utils": "^1.2.1",
 		"yoast/yoastcs": "^3.3.0"
 	},
 	"autoload": {