Remove usage of property hook for DrupalTestCaseTrait::$root

Problem/Motivation

#1792310: Wrong DRUPAL_ROOT with non-standard code structure and #2529170: [PP-1] Remove DrupalKernel::initializeRequestGlobals and replace base_root, base_url and base_path with a service are working to change where app_root and siteDirectory gets managed in DrupalKernel and in tests.

#3589047: Deprecate TestRequirementsTrait::getDrupalRoot() and #3573954: Move common code across Drupal base test suite classes to DrupalTestCaseTrait have made changes that have only landed in main and create a difference in how $root works in tests. This makes refactoring with the aim of a minor release significantly more difficult.

Steps to reproduce

Proposed resolution

Initialize the $root property in a #[Before] hook with high priority so that it's available for all test setup that may be needed.

Deprecation of altering the $root property is postponed to a 12.x version after backporting to 11.x is no longer considered necessary. This can then be done by reintroducing the property hook and removing the internal setUpRoot.

setUpRoot is declared final to prevent overriding, providing a single deprecation path if root is being set directly in contrib tests (this currently doesn't happen in core). With either #1792310: Wrong DRUPAL_ROOT with non-standard code structure or #2529170: [PP-1] Remove DrupalKernel::initializeRequestGlobals and replace base_root, base_url and base_path with a service landing, test would have other methods to affect the application root, even if they can not set $root directly.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet