test: use map for table-driven tests in MultigresCluster controller

Refactors the table-driven tests in `TestMultigresClusterReconciler_Reconcile` to use a map keyed by the test name instead of a slice of structs.

This enforces unique test names and improves the clarity and maintainability of the test suite by explicitly associating the test logic with its description.

Author: fernando-villalba