Refactor assert_routes_registered test helper.

This removes some of the knowledge of router-api paths etc from this
app, and relies on gds-api-adapters for this.

Note: this relies on an unreleased version of gds-api-adapters to be
added in a future commit
Register a backend when registering routes.

The router-api will reject routes pointing at a non-existent backend.
This therefore creates a backend before creating routes.  This is
consistent with other places in our stack where we're registering
routes.