Add `wait_freeing` helper function to ZTS

Add `wait_freeing` helper function to ZTS

Sometimes the ZTS checks freed space just after `zfs destroy snapshot` and
gets an unexpected value because of space being freed asynchronously.
For cases like this add a `wait_freeing` function which blocks until the
pools `freeing` property drops to zero.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #5740