Add a new fwrite test that exercises buffer overflow

Add a new fwrite test that exercises buffer overflow

Exercises fwrite's internal buffer when doing a file operation.
The new test, exercises 2 overflow behaviors:

1. Call fwrite multiple times making usage of fwrite's internal buffer.
   The total number of bytes written is larger than fwrite's internal
   buffer, forcing an automatic flush.

2. Call fwrite a single time with an amount of data that is larger than
   fwrite's internal buffer.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>