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/BackofficeControllerTest.php

@@ -42,4 +42,15 @@ final class BackofficeControllerTest extends AbstractControllerWebTestCase
 
         self::assertResponseRedirects(\sprintf('/backoffice/season/%s', $quiz->season->seasonCode));
     }
+
+    public function testExportQuizIsDeniedForNonOwner(): void
+    {
+        $this->loginAs('test@example.org');
+
+        $quiz = $this->getQuizByName('Quiz 1');
+
+        $this->client->request(Request::METHOD_GET, \sprintf('/backoffice/quiz/%s/export', $quiz->id));
+
+        self::assertResponseStatusCodeSame(403);
+    }
 }