FSK modem tests

# What does it do?

These tests allow either:

- An FSK transmit modem to feed an FSK receive modem, of the same type, through a telephone line model. BER testing is then used to evaluate performance under various line conditions. This is effective for testing the basic performance of the receive modem. It is also the only test mode provided for evaluating the transmit modem.
- An FSK receive modem is used to decode FSK audio, stored in a file. This is good way to evaluate performance with audio recorded from other models of modem, and with real world problematic telephone lines.

# How does it work?