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

[luzpaz]

/path/to/freecad -M /path/to/git-clone/src/Mod/CAM -c -t CAMTests.TestPathVcarve

Example from FreeCAD/FreeCAD#17960 (comment)

This should probably go into https://freecad.github.io/DevelopersHandbook/technical/automated_testing.html