Add mock client for more extensive unit testing

[Westwooo]

The system on which we can run integration tests against an actual cluster is limited by the available github action runners. We used to run the integrations tests against mock clusters, but this capability was removed as part of the test restructuring. However when building for the other systems we still want to be running some testing. We should add a mock client to do unit tests of cases that are hard to trigger against the server. See here for mocking inspo: couchbaselabs/couchbase-rs#155