bug#17797: 24.3; Include ert-buffer.el in Emacs

[Thorsten Jolitz <tjolitz@gmail.com>]

The library 'ert-buffer.el', which does not seem to have a repository
nor to be included in any package repo, but can easily be found via a
web-search, offers some fundamental extensions to ERT functionality in
Emacs core and should thus be included in core.

In summary, it allows to create ert-Buf objects either from string or
from an existing buffer and run tests comparing:

 - point
 - mark
 - content
 - return-value

Since the buffer is such a core concept of Emacs, comparing buffers
seems like a core task in testing. Library ert-buffer.el implements all
the necessary functionality, so that creating this kind of tests is
really fast, even when (sometimes big and complex) real-world
files/buffers are used for testing.

The functionality of ert-buffer.el might potentially be re-implemented
many times when ERT tests that compare buffers are written. Its choice
of criteria for comparing buffers seems pragmatic, keepings things
simple, and thus making it easy to write buffer-tests.

-- 
cheers,
Thorsten