Enable zfs_snapshot_008_neg and zfs_snapshot_009_pos (#5260)

The zfs_snapshot_008_neg test case does not use nested pools and
can be safely enabled. The zfs_snapshot_009_pos test case is
also passing without modification.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: ChaoyuZhang <zhang.chaoyu@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5260