Router client no longer commits routes by default (plus added test helpers).

This reverses the sense of the option parameter to allow optionally requesting to commit as well.

This is a breaking change, and will be released as a major version bump.

The reason for this is that everywhere we use this at the moment, we're registering more than one route, and therefore using the `skip_commit` option.  It therefore makes sense to make this behaviour the default.

This PR also adds some more test helpers for the router client.