Fix strict type errors in test traits

Problem/Motivation

This is a child issue of #3376057: [META] Add declare(strict_types=1) to all tests. After adding enabling strict types to all tests there were around 3000 errors. Fixing them all in one issue will lead to an enormous merge request that's difficult to review, as per the issue scope guidelines.

Steps to reproduce

Create the rector.php file:

<?php

use Rector\Config\RectorConfig;
use Rector\TypeDeclaration\Rector\StmtsAwareInterface\DeclareStrictTypesRector;

return static function (RectorConfig $rectorConfig): void {
  $rectorConfig->rule(DeclareStrictTypesRector::class);
};

Process all test traits:

composer require rector/rector --dev
php ./vendor/bin/rector process core/tests/Drupal/Tests/Traits
find core/tests/Drupal/Tests -type f -name "*Trait.php" -maxdepth 1 | xargs php ./vendor/bin/rector process
find core -type d -path "*/tests/src/Traits" | xargs php ./vendor/bin/rector process

Proposed resolution

Fix all strict type issues in test traits
Follow up [@todo create follow up issue]

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet