Feature request: allow overriding `ignoredRoutes` in `getTestServer()`

`getTestServer()` hard-codes `ignoredRoutes: null`, which re-enables the default `/favicon.ico` and `/robots.txt` 404 short-circuit for HTTP functions. There's no way for a test to reproduce a production deployment that disables it via `--ignored-routes ""`, so functions that legitimately serve those paths can't be tested through `getTestServer()`.

Proposal: accept an optional options bag that overrides the `getServer` options (starting with `ignoredRoutes`), defaulting to today's behavior:

```ts
const server = getTestServer('myFn', {ignoredRoutes: ''});
```

Backward compatible since the parameter is optional. `ignoredRoutes` is the immediate need; a general options bag would also cover `timeoutMilliseconds` etc.

Version: `@google-cloud/functions-framework` 5.0.5.

定位 getTestServer() 入口点，并检查它如何调用 getServer()，重点关注硬编码的 ignoredRoutes 选项。添加 issue 中描述的可选 options bag，保持当前的默认行为，并验证 ignoredRoutes 可以被覆盖，同时现有调用仍保持兼容。

由索引模型根据 Issue 内容生成。