mediawiki.cookie: Export config via packageFiles

• Add a private setDefaults() method to allow mocking from unit tests.

• Use matching keys between the data export and the API.

• Reduce duplication in the code.

• Access the Sinon stub explicitly instead of via the public path.

• Remove use of the now-redundant QUnit.newMwEnvironment().

Change-Id: I600332cdb738f0b443549948b9f070f3ccfa12aa