Add PTT test button to audio modem configuration

Please add a “Test PTT” button to the PTT tab of the audio modem configuration panel that either puts the radio into transmit for a second or so, or else until pushed again. Getting PTT to work is generally pretty fiddly, and it helps a great deal to have a simple way to tell if you’ve done it.

(Also, I can’t get transmission to work with my audio modem setup and I can’t tell if I’ve got the setup wrong or if it’s a bug.)