Refactor tests: fix PHPCS issues and restructure per review feedback

Ciprian Stavoveirequested to merge
issue/views_exclude_previous-3566115:3566115-test-refactor into 3.x
  • Remove redundant unit test
    • Create base test class with shared setUp
    • Separate service tests from plugin tests
    • Fix PHPCS coding standards violations
    • Add #[RunTestsInSeparateProcesses] attribute for Drupal 11.3+

Closes #3566115

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