Produce VOR navigation signal?

[nardev]

I'm just wondering how complicated would it be to add VOR TX tester?

It's just a "(VHF) band from 108.00 to 117.95 MHz" with "30 Hz reference signal on a subcarrier timed to be in phase with the directional antenna as the latter passes magnetic north".

Check the Wiki

In general, signal generators that are being used to test aviation equipment cost from 3 to 10k EUR and i see no reason why not make just an option in rpitx so that pilot's don't need to fly up to check their nav equipment.