Add 'zfs diff' coverage to the ZFS Test Suite

This change adds four new tests to the ZTS:

• zfs_diff_changes: verify type of changes diplayed (-, +, R and M)
• zfs_diff_cliargs: verify command line options and arguments
• zfs_diff_timestamp: verify 'zfs diff -t'
• zfs_diff_types: verify type of objects (files, dirs, pipes...)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Wren Kennedy <john.kennedy@delphix.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6686