Deprecate TestRequirementsTrait::getDrupalRoot()

Problem/Motivation

After #3573954: Move common code across Drupal base test suite classes to DrupalTestCaseTrait , it should be straightforward to deprecate TestRequirementsTrait::getDrupalRoot() and just access ->root.

Proposed resolution

Do it.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet