[FIX] purchase: fix anglosaxon valuation

Before that, when receiving, then invoicing with price diff, the value in interim account corresponded to the price on the invoice instead of the original valuation one.

Scenario to reproduce the issue:

* create PO with 1 product, FIFO valuation, price=10
* receive PO: the entry in stock interim account has debit=credit=10
* create invoice, with price=15 instead of 10
* the invoice entry should be: payable: C=15 ; price diff: D=5 ; interim account: D=10
  ==> Instead, we had payable: C=20 ; price diff: D=5 ; interim account: D=15 => this was wrong

Also added a test case for the same scenario, verifying the amounts written in the different accounts (and refactored a bit ValuationReconciliationTestCase for easiness)

Closes #27394