FWIW, @SergeyL's answer is great, but here is a slight variant for testing. Note the change in logical or to logical and.
main.c has a main wrapper like this:
#if !defined(TEST_SPI) && !defined(TEST_SERIAL) && !defined(TEST_USB)
int main(int argc, char *argv[]) {
// the true main() routine.
}
spi.c, serial.c and usb.c have main wrappers for their respective test code like this:
#ifdef TEST_USB
int main(int argc, char *argv[]) {
// the main() routine for testing the usb code.
}
config.h Which is included by all the c files has an entry like this:
// Uncomment below to test the serial
//#define TEST_SERIAL
// Uncomment below to test the spi code
//#define TEST_SPI
// Uncomment below to test the usb code
#define TEST_USB