bug#29025: Option to run ERT in quiet mode

[Eli Zaretskii]

> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Date: Fri, 27 Oct 2017 11:51:40 +0200
> 
> When running ERT from command-line with (ert-run-tests-batch ...), it
> prints lots of lines for passed tests. The attached patch adds
> variable `ert-quiet' that lets you omit such non-important lines:
> 
>     (let ((ert-quiet t)) (ert-run-tests-batch ...))
> 
> Of course, default is nil, so there is no change for existing
> invocations. It is fully backward compatible. Users don't have to
> check Emacs version prior to binding this variable: on older version
> there will be no effect, but ERT will work.

Thanks, but please also provide patches for the Texinfo manual and
NEWS.