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

[caugner]

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:

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.