Because auth is done outside of Blackcurrant we need a way to use a common CookieJar in tests. Currently, Blackcurrant invents its own cookie jar, which is not ideal.

For this change, we make it so that when test: true, blackcurrant must also take another property 'fetch'. This will be supplied by test runner such that it has already an associated cookie jar with it.

When test: false the 'fetch' property is optional and should default to native fetch.

