ZTS: checkpoint_discard_busy should use save_tunable/restore_tunable

Instead of using hardwired value for SPA_DISCARD_MEMORY_LIMIT,
use save_tunable and restore_tunable to restore the pre-test state.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #16919