Hi

The ERT library is now a part of EMACS. As a result the ERT library
files currently loaded as a sub-module have been deleted on Github. 

This change modifies .gitmodule to avoid attempting to load ERT.
Changes have been made to testing/README.org to suit.

An insertion has been made to testing/README.org to point out that the
library-of-babel.org file needs ingesting (including a source block to
do so) to enable some tests to pass, and the "ERT reference" now points
to the included ERT info help pages.

Broken dependencies in testing/org-test.el have been removed.

I have also temporarily commented out a small number of broken tests
which between them cause ERT to go off into the weeds and delete a large
section of babel.org. This means that all tests currently pass.

I will investigate the breakages and hopefully provide fixes.

Hope this helps

Martyn