OboeTester needs a way to measure latency when AEC is active

When measuring latency with VoiceComm active the device often adds an AEC.
This will cause some or all of the loopback signal to be cancelled.
In some cases a brief chirp can get through before the AEC adaptive filter converges.


So we should add an option to use alternative signals in place of the pulse train including
* brief chirp
* short white noise pulse
* short FM noise burst using a C/M ratio of sqrt(2)