Add a test helper for Rummager-with-queueing

Rummager returns 202 on POST if a feature flag is enabled, because it
queues the change:

https://github.com/alphagov/rummager/blob/cc0e9c95db9098b0dbc3dd12124d1eef5a153ae7/app.rb#L460-L466

This queueing is disabled in development, so this request returns a
different status code from in production. This difference is significant
for third party publishing (eg HMRC manuals API) where the API:

- PUTs a document to the Publishing API (returning 200 or 201)
- POSTs the document to Rummager (returning 200 in dev, but 202 in production)
- returns a status code to the client

We want to return the status code from the Publishing API to the client,
since that reflects whether or not the content has been successfully
published. We therefore want to test that the status code from Rummager
is not being returned accidentally instead. This test helper allows us
to test this case.