ZTS: clone_001_pos fails in cleanup on busy dataset

ZTS: clone_001_pos fails in cleanup on busy dataset

The "cleanup_all" function in this test calls "zfs destroy" which
fails approximately 30% of the time in our environment due to the
dataset being busy. Since the failure happens during cleanup, the
error is propagated to subsequent tests.

Tested by running the snapshot test group in a loop without seeing
any failures.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #8409