Written with raw grouping, so health checks really are included.

Minimizes issues with incompatible tags from different Varnish versions.
The log was generated using testbackend, and includes ESI requests
and backend health checks. The logs are also smaller now.

The response begins with '<', so it is not necessary to disable the
Varnish XML check.

And add tests for Reason.String() and tx.Reason.

Just pass around Status objects (== int8).