rename letter get_folder_name args

`_now`? why would we ever use a different _now? instead say created_at,
because that's what it'll always be set to, even if we're replaying old
letters. We always set the folder name to when the letter was
created_at, or we might not know where to look to find it.

`dont_use_sending_date` doesn't really tell us what might happen if we
don't use it - the answer is we return an empty string. we ignore the
folder entirely. so lets call it that.

Also, remove use of freeze_gun in the tests, to prove that we don't use
the current time in any calculations. Also add an assert to a mock in
the get_pdf_for_templated_letter test, because we were mocking but not
asserting before, so the tests didn't fail when the function signature
changed.

tests/app/celery/test_letters_pdf_tasks.py

@@ -62,7 +62,10 @@ def test_get_pdf_for_templated_letter_happy_path(mocker, sample_letter_notificat
         letter_branding = create_letter_branding(name=branding_name, filename=logo_filename)
         sample_letter_notification.service.letter_branding = letter_branding
     mock_celery = mocker.patch('app.celery.letters_pdf_tasks.notify_celery.send_task')
-    mocker.patch('app.celery.letters_pdf_tasks.get_letter_pdf_filename', return_value='LETTER.PDF')
+    mock_get_letter_pdf_filename = mocker.patch(
+        'app.celery.letters_pdf_tasks.get_letter_pdf_filename',
+        return_value='LETTER.PDF'
+    )
     get_pdf_for_templated_letter(sample_letter_notification.id)
 
     letter_data = {
@@ -87,6 +90,14 @@ def test_get_pdf_for_templated_letter_happy_path(mocker, sample_letter_notificat
         queue=QueueNames.SANITISE_LETTERS
     )
 
+    mock_get_letter_pdf_filename.assert_called_once_with(
+        reference=sample_letter_notification.reference,
+        crown=True,
+        sending_date=sample_letter_notification.created_at,
+        ignore_folder=False,
+        postage='second'
+    )
+
 
 def test_get_pdf_for_templated_letter_non_existent_notification(notify_api, mocker, fake_uuid):
     with pytest.raises(expected_exception=NoResultFound):