test: introduce preReconcileUpdate hook to simplify test fixtures

Refactors the `MultigresCluster` controller tests to remove anonymous immediate functions (IIFEs) used for initializing struct fields.

Instead of defining complex logic inline within the `multigrescluster` field, this change introduces a `preReconcileUpdate` function hook in the test table. This allows the test cases to define a base object cleanly and apply specific mutations (like clearing finalizers or modifying specs) in a dedicated, readable function step before the reconciliation loop runs.

Author: fernando-villalba