ZTS: normalize on use of sync_pool and sync_all_pools
- Replaces use of manual zpool sync
- Don't use log_must sync_pool as sync_pool uses it internally
- Replace many (but not all) uses of sync with sync_pool
This makes the tests more consistent, and makes searching easier.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #12894