A few days ago I was at a customer to configure some optimization settings on their FAS2020 running ONTAP 7.3.4 . The NetApp was configured without thin provisioning and deduplication.
After configuring some of the volumes with deduplication I noticed that two of the SAN volumes used for VMware where suddenly full for 100% after turning on dedup. I then tried to resize the volume with a few more GB but the space was swallowed immediately to 100%. Then I turned off dedup and resized the volume again with a few GB. And it still uses the allocated space immediately!
The dedupe process creates a snapshot at the beginning, so I asume this filled the volume (due to the fractional_reserve). I asume as well, that you have an empty lun in the testvol, so if you create a snapshot there you don't need additional space for the fractional_reserve, as there is no data in the volume.