Tests should run against mock by default

In order to improve the experience of new developers to the various SDK projects, the SDK should be capable of automatically invoking the majority of its tests without undue effort by a new developer.  Towards this goal, all SDKs should implement logic to implicitly fetch a recent version of CouchbaseMock, and invoke it internally for running tests against whenever no local server is available.  The C SDK generally reflects best practices on how to implement this, with Go and Node.js serving as good secondary sources.