Reset item adjustment total

This is a fix for the line-item specific coupon spec tests. OrderUpdater#recalculate_adjustments causes promotion adjustments to be re-applied to line items, resulting in incorrect order total.
In the case of a line item, the total includes both promo_total and additional_tax_total. The tax component should not be considered while computing the promotion discount. Resetting adjustment_total for the adjustable (so the total doesn't reflect any adjustments) before recomputing the adjustments resolves the issue cleanly.