ZTS: Reduce extra caching in pool_checkpoint (#17268)

Those tests are write-mostly at the nested pool. Considering we have
3 more layers of caching underneath, we can hint ZFS how to use the
memory better by setting primarycache=metadata.

While there, add missing zpool sync after rm in checkpoint_capacity
before we could potentially see the freed space, would not there be
a pool checkpoint.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>