Test the `RouteParameters` passed to `TestRouter`

Test the `RouteParameters` passed to `TestRouter`

`TestRouter` allows us to simply select the `Route` that will be
returned in the next `find_route` call, but it does so without any
checking of what was *requested* for the call. This makes it a
somewhat dubious test utility as it very helpfully ensures we
ignore errors in the routes we're looking for.

Instead, we require users of `TestRouter` pass a `RouteParameters`
to `expect_find_route`, which we compare against the requested
parameters passed to `find_route`.