Validate nested reconcilers at test-time

This is a follow-up for https://github.com/reconcilerio/runtime/issues/578.

**Given** I have a nested reconciler (say `Advice[Sequence[ChildReconciler, ChildReconciler]]`)
**When** I run tests for the outer reconciler (say `Advice`)
**Then** I would like my tests to error (not panic) and to see error messages for any of the reconcilers in the tree (say `ChildReconciler`)

