Adding basic framework for unit testing C code

Currently the test_file_stat.c returns true for
all tests - this commit is for the framework, not
real tests :-)

	modified:   Makefile
	new file:   tests/src/minunit.h
	new file:   tests/src/test_file_stat.c
Add tests for null inputs to file_stat functions.
Fixed the code then to pass the tests.

Introduced error codes in file_stat_error.h

	modified:   Makefile
	modified:   lib/file_stat.h
	new file:   lib/file_stat_error.h
	modified:   src/file_stat.c
	modified:   tests/src/test_file_stat.c