Add basic unit tests for Tpz and Tdc

To run those:

For Tdc without any plugged device: python unittests.py TestTdcSim
For Tpz without any plugged device: python unittests.py TestTpzSim
For Tdc with plugged device: SERIAL=/dev/ttyUSBxx python unittests.py TestTdc
For Tpz with plugged device: SERIAL=/dev/ttyUSBxx python unittests.py TestTpz