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.

Locate the getTestServer() entry point and inspect how it calls getServer(), focusing on the hard-coded ignoredRoutes option. Add the optional options bag described in the issue, preserve today’s default behavior, and verify that ignoredRoutes can be overridden while existing calls remain compatible.

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