[IMP] purchase: Remove dependency to AccountTestCommon

l10n runbot builds are all failing when running at least one test depending of AccountTestCommon bec
- doesn't create a sandboxed testing environnement to manage the multi-currency, multi-company, the
- doesn't setup a testing user then all tests are done using the superuser.
- doesn't provide a fully setup chart of accounts: exchange difference journal is not set, accounts
- is run sometimes at-install.

--task: 2301180