Varnishtest client user agents

[Dridi Boukelmoune]

Greetings,

When I write test cases involving multiple requests that don't need to
come from the same client session I try to make one client per
request. When a client reports something wrong, it's usual very
convenient because I don't have to track down too many events.

When I need to jump from a client error to Varnish logs, I find it
increasingly frustrating to actually need to retrace events in the
absence of an X-Varnish header.

Would it be OK to have `client cNAME` send a `User-Agent: vtest
(cNAME)` or similar to reduce some of it?

This should be for the most part transparent, except for test cases
doing request accounting or other header shenanigans (HPACK maybe?)
that should be trivial to adjust.

Thanks,
Dridi