[ADS131] Fixed offset calibration and implemented self test

src/shared/sensors/ADS131M04/ADS131M04.cpp

src/shared/sensors/ADS131M04/ADS131M04.h

src/shared/sensors/ADS131M08/ADS131M08.cpp

src/shared/sensors/ADS131M08/ADS131M08.h

src/tests/sensors/test-ads131m08.cpp