Verify 'zfs destroy' will unshare the dataset

Verify 'zfs destroy' will unshare the dataset

This change adds a new test case to the zfs-test suite to verify that
when 'zfs destroy' is used on a shared dataset, the dataset will be
unshared after the destroy operation completes.

Reviewed by: loli10K <ezomori.nozomu@gmail.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #7941