Generate AHRS test dataset

[bendyer]

The last I/O board recordings use a different packet format, so they'll need to be converted to use the new one (including accelerometer readings divided by 2). Take a ~2 minute sample and store the data in the test directory so accuracy tests can be included in the unittest script.

The AHRS loop will need the facility to read I/O board streams from files rather than the UART buffers via DMA, as well as the facility to output state packets to a file/stdout.