Implement led test mode.

[kareltucek]

Closes UltimateHackingKeyboard/firmware#925.

Not tested on uhk60. For some reason agent can't find it.

[mondalaci]

Please extend the implementation with the following behavior:

• Don't emit any scancodes when in test mode
• After switching to the text mode and all LEDs are enabled, when pressing the first key, disable all LEDs on both sides except the LED of the pressed key

[kareltucek]

After switching to the text mode and all LEDs are enabled, when pressing the first key, disable all LEDs on both sides except the LED of the pressed key

Please no, unless there is a good reason to justify that requirement.

It would greatly increase the complexity of the entire solution.

[mondalaci]

Ok, it's not a hard requirement.

Can you suppress the scancodes, though?

[kareltucek]

Yes, working on it!

[kareltucek]

I am implementing the suppression for uhk80 only at the moment.

If we want it to work on uhk60, I can do that, but it needs a separate solution and so I would prefer not to do it now given the mentioned testing issues.

Is that ok?

[kareltucek]

(Done.)

[mondalaci]

It'd be favorable if the keys could be lit up on keypress, not on key release. (Only if this can be implemented quickly.)

Some keys don't light up the relevant LEDs, including ScrLk, Pause, Ins, Del, ,, ., /.

[kareltucek]

(Remains to test / finish UHK60 -> telegram.)