Update testing documentation

[contsili]

This PR will remain as a draft since we need to decide what the use of ft_test will be.

[robertoostenveld]

I suggest splitting "release+QC" and "testing" over two pages, and to use more or less this outline:

Software release and quality control

Releases

Nightly testing

Testing during development

This points to the Testing page

Testing

Why

Running existing tests

Here give the example for finding them

Extending existing tests

In general you should not do this because ...

However, if you ...

Making new tests

To make sure that others can find and run your test, please specify the HEADERS and send the data ...

Working with data

no

public

private