Launch the DevTools server from source in integration tests

This will allow for testing end to end changes that span the DevTools server and the DevTools app. In practice, this is not practical to do until DevTools is moved into the Dart SDK.

This is a follow up issue to https://github.com/flutter/devtools/issues/9176.

首先阅读后续 issue #9176 和 integration-test 设置；payload 未指定具体文件或入口点。确认当前如何启动 DevTools 服务器和应用，然后将完成条件定义为从源代码启动服务器，以便在迁移 Dart SDK 后测试跨越两个组件的端到端更改。

由索引模型根据 Issue 内容生成。