Add 'zpool split' coverage to the ZFS Test Suite

This change adds five new tests to the ZTS:

• zpool_split_cliargs: verify command line options and arguments
• zpool_split_devices: verify zpool split accepts a device list
• zpool_split_encryption: verify zpool can split encrypted pools
• zpool_split_props: verify zpool split can set property values
• zpool_split_vdevs: verify vdev layout when splitting the pool

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7409