test: refactor TableGroup controller tests for consistency and hygiene

Refactors the `TableGroup` controller tests to align with the patterns established in the `MultigresCluster` controller test suite.

Changes include:
- Splitting `TestTableGroupReconciler_Reconcile` into `_Success` and `_Failure` suites for better organization.
- Converting test cases to a map-based structure to enforce unique names and order independence.
- Adding `t.Parallel()` to all tests to enable concurrent execution.
- Replacing `context.Background()` with `t.Context()` for proper test lifecycle management.
- Updating helpers to use the `testing.TB` interface for broader reusability.
- Introducing a `preReconcileUpdate` hook and `skipCreate` flag to simplify test fixtures and remove imperative logic from the test table.
- Enforcing strict error handling in all setup and validation steps to prevent silent failures.

Author: fernando-villalba