Fix res_partner tests after OCA/OCB partner's VAT validation update

CHANGELOG:

### Changed
- [#430](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/430) Use a valid VAT in res_partner tests

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Even if we don't use this updated OCA/OCB version in our CI:

After this commit: https://github.com/OCA/OCB/commit/d5040d85e6dcf6d4d17b3e3b38d74e14c16e4b60

Tests would fail in our CI gitlab pipeline with these error:

odoo.exceptions.ValidationError: ("\nThe VAT number [00470223B] for partner [test] does not seem to be valid. \nNote: the expected format is 'CC##' (CC=Country Code, ##=VAT Number)", None)

Anyway, we should have these tests updated because our ODOO does not validate NIFS without VAT format (starting with a two-digit country code).