Bell MF tone generation and detection tests

# What does it do?

These tests are fashioned after those on the CM7291 test tape from Mitel. Those tests are for DTMF, rather than Bell MF, but make a fair starting point for a set of meaningful tests of Bell MF.

These tests include conversion to and from A-law. It is assumed the distortion this produces is comparable to u-law, so it should be a fair test of performance in a real PSTN channel.