Document the scope of Gaffer's testing framework

[t616178]

Specifically, provide a description of the test coverage for the different combinations of parameters, settings and loading mechanisms that can be used with a Gaffer graph.

[GCHQDeveloper314]

This is something which could be done as part of the Gaffer docs. A page covering this could be placed under the development guide pages. This issue can be transferred to the gaffer-doc repo.