test: rename cluster field to multigrescluster in controller tests

Renames the `cluster` field in the `MultigresCluster` controller test cases to `multigrescluster`.

The variable name `cluster` is ambiguous in this context, as it could refer to the underlying Kubernetes cluster or the `MultigresCluster` custom resource. Using the explicit name `multigrescluster` improves clarity and avoids potential confusion regarding which entity is being referenced in the test fixturs.

Author: fernando-villalba