Here's your mistake.
cp /testfile /dev/fd0
you need to copy the file to where the floppy is mounted (mount point). In your case as you described /mnt
In case you aren't aware, it is typical to create a subfoler within /mnt for each removable or mounted filesystem.
For example /mnt/cdrom
for a cd-rom drive, /mnt/floppy
for your floppy, etc. That would make your fstab entry
/dev/fd0 /mnt/floppy auto,users, 0 0
You said you have no problem reading files off your CD-Rom. Where is that mounted?