Create utility function for creating a stub statefulset for unit tests

Currently we create a statefulset inside of one of the test files [cockroach_statefulset_test.go](https://github.com/cockroachdb/cockroach-operator/pull/519/files#diff-a1ba08c59a33ba5c7ebaa3de9e854a9df7c9b433804ab242c304afea521708e3R134) this should be pulled out so that all unit tests can use it as needed. There's also a pod creation function that needs factoring out as well.