test: split MultigresCluster reconcile tests into success and failure suites

Refactors the monolithic `TestMultigresClusterReconciler_Reconcile` function into two separate test functions: `TestMultigresClusterReconciler_Reconcile_Success` and `TestMultigresClusterReconciler_Reconcile_Failure`.

This separation clarifies the intent of each test case and simplifies the test table structure by removing the need for an `expectError` field. It also introduces a `setupFixtures` helper to share common initialization logic while ensuring test isolation.

Author: fernando-villalba