tests: fix leak in `test_util_file` ; found by cppcheck

Which now seems to fail the build.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

tests/test_util_file.c

@@ -106,6 +106,7 @@ static void stat_and_cat(const char *file)
 	buf[sb.st_size] = '\0';
 	printf("file[%s], size=%d, contents=%s\n", file, (int)sb.st_size, buf);
 	free(buf);
+	close(d);
 }
 
 int main(int argc, char **argv)