Add functional test coverage for placing a Quicktabs block via Block Layout

Problem/Motivation

Quicktabs instances are intended to be exposed as blocks and placed through Drupal's Block Layout UI, but the current test coverage does not verify that workflow. Existing functional coverage tests creating and deleting a Quicktabs instance, but not placing the derived Quicktabs block in a region and confirming it renders as expected.

Proposed resolution

Add functional test coverage for placing a Quicktabs block through Block Layout after creating a Quicktabs instance. Verify that the Quicktabs block is available for placement, can be assigned to a region, and renders the expected tabs on the page.