Fix failing dynamic property adminUser and Use FileExists enum in test

Problem/Motivation

The tests are failing, let's fix them.

Unsilenced deprecation notices (1)

  1x: Creation of dynamic property Drupal\Tests\config_translation_po\Functional\ConfigTranslationPOTest::$adminUser is deprecated
    1x in ConfigTranslationPOTest::testImportExport from Drupal\Tests\config_translation_po\Functional

Remaining self deprecation notices (2)

  2x: Passing the $fileExists argument as an integer to Drupal\Core\File\FileSystem::copy() is deprecated in drupal:10.3.0 and is removed from drupal:12.0.0. Use \Drupal\Core\File\FileExists enum instead. See https://www.drupal.org/node/3426517
    2x in ConfigTranslationPOTest::testImportExport from Drupal\Tests\config_translation_po\Functional

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes