test: address PR review feedback

- Scope AbstractControllerWebTestCase::getCandidate/getQuizByName by
  season code (both Candidate and Quiz are only unique per season, not
  system-wide) and add a CandidateRepository regression test guarding
  against same-named candidates in different seasons
- Add missing entityManager->clear() before verifying DB state after a
  POST in PrepareEliminationControllerTest and QuestionBankControllerTest
- Add non-owner denial tests for BackofficeController::exportQuiz and
  QuizQuestionController::edit/reorder, which had IsGranted checks with
  no test coverage

tests/Controller/Backoffice/PrepareEliminationControllerTest.php

@@ -52,6 +52,8 @@ final class PrepareEliminationControllerTest extends AbstractControllerWebTestCa
         $this->assertTrue($response->isRedirect());
         $this->assertStringContainsString('/backoffice/elimination/', (string) $response->headers->get('Location'));
 
+        $this->entityManager->clear();
+        $quiz = $this->getQuizByName('Quiz 1');
         $elimination = $this->entityManager->getRepository(Elimination::class)->findOneBy(['quiz' => $quiz]);
         $this->assertInstanceOf(Elimination::class, $elimination);
         $this->assertArrayHasKey('Tom', $elimination->data);