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/include/blkdev.shlib

@@ -595,7 +595,7 @@ function list_file_blocks # input_file
 	else
 		AWK='awk'
 	fi
-	log_must zpool sync -f
+	sync_all_pools true
 	zdb -dddddd $ds $objnum | $AWK -v pad=$((4<<20)) -v bs=512 '
 	    /^$/ { looking = 0 }
 	    looking {