Add subsection to automated testing on 'how to trigger unittests locally in the CLI'

```shell
/path/to/freecad -M /path/to/git-clone/src/Mod/CAM -c -t CAMTests.TestPathVcarve
```
Example from https://github.com/FreeCAD/FreeCAD/pull/17960#issuecomment-2524575663

This should probably go into https://freecad.github.io/DevelopersHandbook/technical/automated_testing.html