Add a CRUD test (create/fetch/update/delete) for the MultiCurrency
class, which had no test coverage yet, plus coverage of its rate
handling (addRate/updateRate/getRate/fetchAllCurrencyRate) and of
the static helpers used throughout Commande/Propal/Facture creation
when a document uses a foreign currency (getIdFromCode,
getIdAndTxFromCode, getAmountConversionFromInvoiceRate).
The module is auto-activated in setUpBeforeClass() if not already
enabled, following the same pattern as OpensurveysondageTest,
WorkstationTest and DolresourceTest (activation is real and persists
after the test run, it is not undone by the rollback in
tearDownAfterClass - see comment in setUpBeforeClass() for why).
Uses ISO 4217's reserved test currency code XTS to avoid colliding
with a real currency.
