Composer: use wp-cli-tests ^2.1

... as that version contains the new WPCliCS PHPCS standard.

Author: jrfnl

composer.json

@@ -18,7 +18,7 @@
         "wp-cli/db-command": "^1.3 || ^2",
         "wp-cli/entity-command": "^1.3 || ^2",
         "wp-cli/extension-command": "^1.2 || ^2",
-        "wp-cli/wp-cli-tests": "^2.0.7"
+        "wp-cli/wp-cli-tests": "^2.1"
     },
     "config": {
         "process-timeout": 7200,