Update filemonkeytest.c

[adiwillhit]

Changes in :

1. "argc" to "argc -1" : changed error message to show the actual number of arguments received.
2. Add a read_rand file check to prevent crashes if it fails to open.
3. Prints an error message before exiting if any metric file fails to open.
4. I used srand((unsinged int ) time (NULL) instead of srand(rand()) for better randomness.
5. I also used snprintf() instead of sprintf() to avoid buffer overflows.