test: refactor cluster-handler integration tests and standardize controller setup

Refactors the integration tests for `multigrescluster` and adds new tests for `tablegroup` to match the style and patterns used in the `resource-handler`.

Changes:
- **Controller Setup:** Updated `MultigresClusterReconciler` and `TableGroupReconciler` to accept variadic `controller.Options` in `SetupWithManager`. This allows injecting `SkipNameValidation: true` for parallel testing, consistent with the Shard controller.
- **MultigresCluster Tests:** Rewrote `integration_test.go` to remove `gomega`, leverage the internal `testutil` package, and use table-driven tests. Fixed test fixtures to include required `RootPath` and explicit cell assignments.
- **TableGroup Tests:** Added a new `integration_test.go` for TableGroups using the `testutil` pattern.
- **Unit Tests:** Updated `_controller_test.go` files to maintain 100% coverage by exercising the new variadic options logic.

Author: fernando-villalba