When FEATURES="-userpriv test" is set, sys-fs/zfs-9999 will have a 'test' phase failure. This was reported to me first by a user in IRC and second by Chris Reffett in bug #405203. Bug #405203 was closed because the original issue was fixed, so I am opening this bug to track progress on it.
Unfortunately, it is impossible to do testing when ZFS is used as a rootfs. I added RESTRICT=test to the ebuild several months ago, so I am closing this.