Tests: Fix for currency spec

[marki1an]

This PR aim to resolve this issue:

Error:  Failures: 
Error:    CurrencySpec.PBS should return currency rates:38 Condition not satisfied:
response.rates?.size() > 0
|        |      |      |
|        null   null   false
org.prebid.server.functional.model.response.currencyrates.CurrencyRatesResponse(active:true, source:http://tc-9DxGcinQ:1080/currency, fetchingIntervalNs:900000000000)