Refactor tests code via Rector

Problem/Motivation

We completed using Rector on core/tests code with withPreparedSets(typeDeclarations: true).

Now we can try running it on every test class in core, including modules, and cleanup the entire test codebase.

Proposed resolution

Use rector with type coverage on all tests classes in core; for now skip the TypedPropertyFromCreateMockAssignRector rule that we learned requires manual adjustments.

Install rector

$ composer require --dev rector/rector palantirnet/drupal-rector

Configure rector.php file

declare(strict_types=1);

use Rector\TypeDeclaration\Rector\Class_\TypedPropertyFromCreateMockAssignRector;

return Rector\Config\RectorConfig::configure()
  ->withBootstrapFiles([
     'vendor/palantirnet/drupal-rector/config/drupal-phpunit-bootstrap-file.php',
  ])
  ->withPaths([
    'core/tests',
    'core/modules/**/tests/src',
  ])
  ->withSkip([
    '*/*ixture*/*',
    '*/Annotation/Doctrine/*',
    'core/tests/Drupal/Tests/TestTools/ErrorHandler/DrupalDebugClassLoaderTest.php'
  ])
  ->withPreparedSets(typeDeclarations: true)
  ->withSkip([
    TypedPropertyFromCreateMockAssignRector::class,
  ])
  ->withImportNames(
    importDocBlockNames: false,
    importShortClasses: false,
    removeUnusedImports: false,
  );

Run rector
$ ./vendor/bin/rector

Fix code style
$ composer phpcbf

Update baseline
$ ./vendor/bin/phpstan analyse -c core/phpstan.neon.dist --generate-baseline=core/.phpstan-baseline.php --memory-limit=2G

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet