Write a test for mb_format.c about mb-system

[schwehr]

cvt_to_nix_path should only be defined if #ifdef WIN32 and it should be a void return type. e.g.

#ifdef WIN32
void cvt_to_nix_path(char *path) {
	/* Replace back slashes by slashes and trim first two chars in paths like "C:/path" */
	const size_t len = strlen(path);

	if (len == 0)
		return;
	for (int k = 0; k < len; k++)
		if (path[k] == '\\')
			path[k] = '/';

	if (path[1] == ':') {
		for (int k = 0; k < len - 2; k++)
			path[k] = path[k + 2];
		path[len - 2] = '\0'; /* Make sure it's null terminated */
	}
}
#endif

[schwehr]

#125 adds a test for utilities/mbformat.c which covers some of mb_get_format(), mb_format(), and mb_format_description(). However, it would still be better to have direct testing of all the functions in mbio/mb_format.c.