Issue #3529128: Setup PHPUnit testing command

For #3529128

• Adds a new composer run phpunit command
• Adds test suite splitting and parallelisation to GitLabCI for PHPUnit

Testing instructions

• Try the different methods of running composer run phpunit described in docs/testing/phpunit.md
• Add a Build test locally to tests/src/Build/DeleteMeTest.php

<?php

declare(strict_types=1);

namespace Drupal\Tests\canvas\Build;

use Drupal\BuildTests\Framework\BuildTestBase;

// This file exists only to verify ForbiddenTestTypesSniff catches Build tests.
// Delete it once confirmed.
class DeleteMeTest extends BuildTestBase {

  public function testNothing(): void {}

}

• Run composer run phpcs and verify this raises an error