vmod-vtc for varnishtest users

[Dridi Boukelmoune]

Greetings testers,

I would like to draw your attention to a pull request that turned into
a discussion (instead of being merged in) regarding a future VMOD for
out-of-tree usage of varnishtest:

https://github.com/varnishcache/varnish-cache/pull/2276#issuecomment-297406551

If you are a VMOD developer, or someone who writes varnish-related
tools and use varnishtest to run your test suite, now is the time to
let us know what would make testing more convenient with a vanilla
install of Varnish.

If you are not extending Varnish, you may also find varnishtest more
and more useful to test your VCL as releases go by. If you also use
varnishtest in this context, you may have run into shortcomings, let
us know if that's the case.

You may now think that using such a VMOD to test your production VCL
wouldn't be appropriate. I will intentionally be vague and answer that
not necessarily. If you aren't familiar with varnishtest yet, I will
as usual recommend to get acquainted to it. It is even more accessible
now that Guillaume put up a manual for the VTC syntax too. You can
always learn from existing test cases.

We will already ship a bunch of functions that we use in the varnish
test suite that are considered useful outside of varnish too. There
may be use cases you ran into, do not hesitate to suggest more
fixtures.

If time permits, and if it is put on the agenda, we will discuss this
next Monday. However this should be part of the September release
and it shouldn't be a problem suggest new goodies later too.

Best,
Dridi