makefile: Add cunit-tests/log-tests/clean-tests commands for convenience

During fbc development I end up doing this very often:
	cd tests
	make cunit-tests FBC='~/fbc/bin/fbc -i ~/fbc/inc'
	make   log-tests FBC='~/fbc/bin/fbc -i ~/fbc/inc'
	make clean

The new commands make this a little easier because it's no longer needed
to enter the long commands manually/copypaste/etc.