Set item.pre_tax_amount in memory, not in DB
Add tax_category stubs to ItemAdjustments spec, now that OrderContents copies over tax categories
Remove all_adjustments test that assumes its still an AR association
Implement ItemAdjustments#update_adjustments, since some tests require it
Use OrderContents#add within checkout_spec to setup env correctly
Remove unnecessary testing setup + code in order/checkout_spec for update_totals
WIP: tax_rate_spec refactoring