Update testing intro doc

[mwichmann]

This does not affect SCons at all - only updates the document (which is eventually duplicated on the Wiki) about making use of the SCons test framework.

It's not going to be easy to review in the classical way, maybe just scan through the full doc and see if it makes sense this way?

https://scons.org/guidelines.html

Contributor Checklist:

• [ ] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [ ] I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• [X] I have updated the appropriate documentation