utest: serial_v2: add depends on RT_USING_SERIAL_V2

[unicornx]

Add dependent of RT_USING_SERIAL_V2 in Kconfig to make sure utest for serial_v2 is only available when RT_USING_SERIAL_V2 is enabled.

Otherwise menuconfig allow people enable UTEST_SERIAL_TC but building of utestcases failed. It looks weird and better to be fixed.

[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

---

🛠 操作步骤 | Steps

1. 前往 Actions 页面 | Go to the Actions page
   点击进入工作流 → | Click to open workflow →

2. 点击 Run workflow | Click Run workflow

• 设置需排除的文件/目录（目录请以"/"结尾）
  Set files/directories to exclude (directories should end with "/")
• 将目标分支设置为 \ Set the target branch to：dev-utest-serialv2
• 设置PR number为 \ Set the PR number to：10784

3. 等待工作流完成 | Wait for the workflow to complete
   格式化后的代码将自动推送至你的分支。
   The formatted code will be automatically pushed to your branch.

完成后，提交将自动更新至 dev-utest-serialv2 分支，关联的 Pull Request 也会同步更新。
Once completed, commits will be pushed to the dev-utest-serialv2 branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!