Bug#973393: truncate less of the backtrace during failing ert tests

[Nicholas D Steeves <nsteeves@gmail.com>]

Package: dh-elpa
Version: 2.0.4
Severity: normal

Hey team,

Thomas Koch added a nice workaround for truncated backtraces at:

  https://wiki.debian.org/Teams/DebianEmacsenTeam/Tips

that workaround is d/elpa-test:

  ert_eval = (setq ert-batch-backtrace-right-margin 500)

and I wonder if it should be activated by default, in the spirit of
Policy §4.9 "The package build should be as verbose as reasonably
possible".  Speaking for myself, I would find it helpful, especially
for the rare corner cases where only noninteractive --batch ert tests
will trigger a failure.  Also, I've been asked for untruncated
backtraces by various upstreams.

The only potential issue I can think of is that the reproducible and
DebCI build logs will have then have long lines, but I feel like the
benefit outweighs this consideration.  A possible, though not ideal,
solution to this potential issue might be to word-wrap the backtrace,
but that functionality should probably be enabled in upstream Emacs.

Let's consider setting `ert-batch-backtrace-right-margin` to a large
value in the meantime.

Regards,
Nicholas