[SPIDriver] Tests & minor refactoring

src/shared/drivers/Xbee/Xbee.h

src/shared/drivers/spi/MockSPIBus.h

src/shared/drivers/spi/SPIBus.h

src/shared/drivers/spi/SPIBusInterface.h

src/shared/drivers/spi/SPIDriver.h

src/shared/drivers/spi/SPITransaction.cpp

src/shared/drivers/spi/SPITransaction.h

src/tests/catch/spidriver/FakeSPIBus.h

src/tests/catch/spidriver/test-spidriver.cpp