ZTS: Make do_vol_test() more deterministic (#16379)

• Explicitly disable compression since mkfile uses a zero buffer.
  • Explicitly sync file systems instead of waiting for timeout.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>