Non consistant assertExactQueries in memberships tests

Visible on the dev setup (Erwan, Valentin):

• Works running manage.py test arkindex.documents.tests.tasks.test_corpus_delete
• Fail running manage.py test

In some conditions, the Corpus ContentType is cached and is not part of the exact SQL query.

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FAIL: test_run (arkindex.documents.tests.tasks.test_corpus_delete.TestDeleteCorpus)
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Traceback (most recent call last):
  File "/home/lucidiot/dev/ark/backend/arkindex/documents/tests/tasks/test_corpus_delete.py", line 123, in test_run
    corpus_delete(self.corpus.id)
  File "/home/lucidiot/dev/ark/backend/arkindex/project/tests/__init__.py", line 70, in __exit__
    self.test_case.assertEqual(expected_sql, actual_sql)
AssertionError: 'SELE[375 chars];\n\nDELETE\nFROM "documents_allowedmetadata"\[5438 chars]uuid' != 'SELE[375 chars];\n\nSELECT "django_content_type"."id",\n     [5710 chars]uuid'
  SELECT "documents_corpus"."created",
         "documents_corpus"."updated",
         "documents_corpus"."id",
         "documents_corpus"."name",
         "documents_corpus"."description",
         "documents_corpus"."repository_id",
         "documents_corpus"."public"
  FROM "documents_corpus"
  WHERE "documents_corpus"."id" = '07df48d1-5a28-428d-a7b8-8a17e14bda82'::uuid
+ LIMIT 21;
+ 
+ SELECT "django_content_type"."id",
+        "django_content_type"."app_label",
+        "django_content_type"."model"
+ FROM "django_content_type"
+ WHERE ("django_content_type"."app_label" = 'documents'
+        AND "django_content_type"."model" = 'corpus')
  LIMIT 21;