Add bad_backend macro and use it instead of ${bad_ip}/port,
should shave a few seconds here and there.
While here also replace some unneeded ${bad_ip} entries.

If you give multiple -f arguments, the resulting VCL's will be
"boot0", "boot1", ... "boot668", "boot"

And the last one will be the active one.

The plan is to also allow both specification of an (optional) label and
which VCL is to be the active one, but I want to think about if we should
shoehorn that into -f or use a different option letter.

all other modes than -d.

Allow -f '' as an override for people who really want this.

Fixes: #2167

Fixes #1704

processed until after we switched VCL.

Fixes #2052

Fixes #2078

general cleanup and polishing.

Inherited from out-of-tree coverage scripts.

When a `distcheck` fails, the $(distdir) is not removed and this is how
we can collect test results. This directory is also left untouched by
the `distclean` target run from `autogen.des`.

So if the next commit checked by vtest fails before running inside the
$(distdir), it will report failures from the previous run. To avoid that
we can use $(am__remove_distdir) that deals with permissions but may not
be stable API.

Instead of randomly looking for the log files, look for them where we
expect them to land. For instance, after a git pull, a distclean
operation may not remove the dist directory if Varnish's version
changed because it would reconfigure itself on the fly.

Instead of reading the output of `make distcheck`, look for failing
tests in the dist directory.

to avoid any memory leaks.

Closes	#2177