[TEST] add tests for source currencies query parameter for path find route

Please clarify:

> I would prefer to have all responses in the fixtures, not in testutils.

Do you want 

```fixtures.RESTErrorResponse({
      type: 'transaction',
      error: 'tefMAX_LEDGER',
      message: 'Ledger sequence too high.'
    }))```

in each test's fixture file instead of a global file?