Used in FLAT tests to register a flow that mocks an upstream response.
<backend-flow request="id"><!-- flow actions --></backend-flow>
When upstream requests are carried out, their request IDs are matched against the
request= attribute of the backend flow. On match, the backend flow is executed instead of the actual HTTP request.
If at least one backend flow is registered, no HTTP request will be sent. If no backend flow matches, the request will fail.