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

tests/zfs-tests/tests/functional/deadman/deadman_sync.ksh

@@ -70,7 +70,7 @@ log_must file_write -b 1048576 -c 8 -o create -d 0 -f $mntpnt/file
 sleep 10
 
 log_must zinject -c all
-log_must zpool sync
+sync_all_pools
 
 # Log txg sync times for reference and the zpool event summary.
 if is_freebsd; then

tests/zfs-tests/tests/functional/deadman/deadman_zio.ksh

@@ -83,7 +83,7 @@ log_must zinject -d $DISK1 -D10000:1 $TESTPOOL
 log_must eval "dd if=/$mntpnt/file1 of=/dev/null bs=1048576 &"
 sleep 10
 log_must zinject -c all
-log_must zpool sync
+sync_all_pools
 wait
 
 # 5. Verify a "deadman" event is posted.  The first appears after 5