Check for what DVLALetterTemplate is called with

Extends the test to make sure that the thing that builds each line of
the file is getting called with the right template, personalisation and
numeric ID. Will be helpful the more complicated the call to the
template gets.

tests/app/celery/test_tasks.py

@@ -971,17 +971,29 @@ def test_build_dvla_file(sample_letter_template, mocker):
     create_notification(template=job.template, job=job)
     create_notification(template=job.template, job=job)
 
-    mocked = mocker.patch("app.celery.tasks.s3upload")
-    mocker.patch("app.celery.tasks.LetterDVLATemplate.__str__", return_value="dvla|string")
+    mocker.patch("app.celery.tasks.random.randint", return_value=999)
+    mocked_upload = mocker.patch("app.celery.tasks.s3upload")
+    mocked_letter_template = mocker.patch("app.celery.tasks.LetterDVLATemplate")
+    mocked_letter_template_instance = mocked_letter_template.return_value
+    mocked_letter_template_instance.__str__.return_value = "dvla|string"
     build_dvla_file(job.id)
 
-    file = "dvla|string\ndvla|string\n"
+    mocked_upload.assert_called_once_with(
+        filedata="dvla|string\ndvla|string\n",
+        region=current_app.config['AWS_REGION'],
+        bucket_name=current_app.config['DVLA_UPLOAD_BUCKET_NAME'],
+        file_location="{}-dvla-job.text".format(job.id)
+    )
 
-    assert mocked.called
-    mocked.assert_called_once_with(filedata=file,
-                                   region=current_app.config['AWS_REGION'],
-                                   bucket_name=current_app.config['DVLA_UPLOAD_BUCKET_NAME'],
-                                   file_location="{}-dvla-job.text".format(job.id))
+    # Template
+    assert mocked_letter_template.call_args[0][0]['subject'] == 'Template subject'
+    assert mocked_letter_template.call_args[0][0]['content'] == 'Dear Sir/Madam, Hello. Yours Truly, The Government.'
+
+    # Personalisation
+    assert mocked_letter_template.call_args[0][1] is None
+
+    # Named arguments
+    assert mocked_letter_template.call_args[1]['numeric_id'] == 999
 
 
 def test_build_dvla_file_retries_if_all_notifications_are_not_created(sample_letter_template, mocker):