Delete CorpusWorkerVersion when deleting a corpus

Closes #882 (closed)

arkindex/documents/tests/tasks/test_corpus_delete.py

 from django.db.models.signals import pre_delete
 
-from arkindex.dataimport.models import DataImportMode, Repository, RepositoryType, WorkerVersion
+from arkindex.dataimport.models import CorpusWorkerVersion, DataImportMode, Repository, RepositoryType, WorkerVersion
 from arkindex.documents.models import Corpus, Element, EntityType, MetaType, Transcription
 from arkindex.documents.tasks import corpus_delete
 from arkindex.project.tests import FixtureTestCase
@@ -109,6 +109,9 @@ class TestDeleteCorpus(FixtureTestCase):
         )
         cls.page.add_parent(cls.vol)
 
+        # Generate some CorpusWorkerVersions automatically
+        CorpusWorkerVersion.objects.rebuild()
+
     def test_run(self):
         receivers = pre_delete.receivers
 
@@ -156,6 +159,8 @@ class TestDeleteCorpus(FixtureTestCase):
         self.assertEqual(ts.text, 'hi')
         self.assertEqual(ts.confidence, 0.75)
 
+        self.assertEqual(self.corpus2.worker_version_cache.count(), 1)
+
     def test_run_top_level_type(self):
         receivers = pre_delete.receivers