Add CN-testSite server entry

[xhstoyea]

为中国玩家提供一个公开的BE版测试场地

[github-actions]

Mindustry Server List Validation

❌ JSON parse errors

• servers_be.json: Expected ',' or '}' after property value in JSON at position 296

Fix the JSON syntax above and push an update to this PR; pings were not run.

_{Last updated: 2026-08-05T12:30:29.363Z}