We only care about non-cart orders and skipping carts, saves PostgreSQL
query planner to go through thousands of records in production use cases
(my food hub).
We go from
```sql
-> Index Scan using index_spree_orders_on_customer_id on spree_orders (cost=0.42..12049.45 rows=152002 width=15) (actual time=0.015..11.703 rows=13867 loops=1)
```
to
```sql
-> Index Scan using index_spree_orders_on_customer_id on spree_orders (cost=0.42..12429.46 rows=10802 width=15) (actual time=0.025..17.705 rows=9954 loops=1)
```
As is, `payment_total` is only increased after successfully processing
a payment and never updated. This inconsistency breaks
`CustomerWithBalance` which relies on it.
Needless to say that if we keep this denormalized column, we better make
it consistent. I investigated current Spree's master branch (709e686cc0)
and they also realized it was broken. Now `Payment` runs the following
from the `after_save` `update_order` callback.
```rb
order.updater.update_payment_total if completed? || void?
```
I also took the chance to rearrange tests a bit.
