Always create assets in test env `rake test_app`

Build in extensions broke because Spree::Frontend and Spree::Backend are
not defined when running `rake test_app` within extension gem context

Fixes #3441
Remove duplicated order dir under core spec/models
Add failing spec for promo affecting taxes
User tax calculator for tax_rates on order totals spec
Adjust tax when order has promotions

Had to make tax adjustment mandatory, open and the DefaultTax calculator
aware of promotion adjustments
In taxes only compute promo discounts when they exist
Dont reload records on update_adjustments

Update CreateAdjustment spec to still work as expected