Feature: Add keyboard/switch test mode

[NuclearMonster]

Hello,
Please add a test mode to PEG.

This could be similar to the one in VIA where you can test either the layout or the matrix in order to have full test coverage of a keyboard build.

Thank you for your work

[daysgobye]

Great idea I'll look into the challenges of doing this I have already written a tester at https://toolbox.peg.software/tester/

So probably would not be that hard to move that over