Tweak testing docs

[bobturneruk]

• Add (if needed) info on baselines
• Explain what all the different folders of data are for
• Explain what we feel we achieve by testing
• Comment out "furloughed" code e.g. network of individuals

[github-actions[bot]]

Heads up @magicicada @bobturneruk @aflag @WPettersson @alex-konovalov @may1066 @mrow84 - the "Simple Network Sim" label was applied to this issue.

[bobturneruk]

The point of commenting out the code is so that we get more accurate reports of test coverage.