dev/core#1983 Fix to tax calculation on multi-line-item

This is similar to https://github.com/civicrm/civicrm-core/pull/18284 - it differs in that the totals are calculated by iterating
through the line item array afterwards, rather than expecting the 'getLine' function to calculate totals. Some
obvious follow ups suggest themselves but I will look against master.

This is difficult to test (Karin gave it a really good shot) because of the weird way it's calculated in Main and thenn
used in Confirm. Cleanup should resolve the testability issue too