[#1873] Update testing styleguide.


Problem description
===================

We made a few changes in the testing styleguide so we need to update the document.

Especially the fact that system tests are now merged with unit and integration tests.


Changes description
===================

I change the info that system tests needs to be separate.

I have also updated the guide with generic page.


How to try and test the changes
===============================

reviewers @alibotean 

Check that changes make sense.

Page is published here http://styleguide.chevah.com/testing.html