ZTS: normalize on use of `sync_pool` and `sync_all_pools`

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