Generate Vodafone link test sequence nos in proxy

Previously the Celery task to trigger a link test had to know about
the special case of a sequence number for Vodafone. Since we're about
to change the client to perform multiple tests it makes sense to give
it the knowledge of how to generate number itself.

Note that we have to import the db inline to avoid a circular import,
since this module is itself imported by app/__init__.py.

Other invocations of the Vodafone client use stored sequence numbers
from the DB, which are called "message numbers" in that context. Since
the two use cases are very different (even the names are different!),
having them in two places shouldn't cause any confusion.

app/clients/cbc_proxy.py

@@ -5,6 +5,7 @@ import boto3
 import botocore
 from flask import current_app
 from notifications_utils.template import non_gsm_characters
+from sqlalchemy.schema import Sequence
 
 from app.config import BroadcastProvider
 from app.utils import DATETIME_FORMAT, format_sequential_number
@@ -79,7 +80,6 @@ class CBCProxyClientBase(ABC):
     def send_link_test(
         self,
         identifier,
-        sequential_number
     ):
         pass
 
@@ -158,7 +158,6 @@ class CBCProxyOne2ManyClient(CBCProxyClientBase):
     def send_link_test(
         self,
         identifier,
-        sequential_number=None,
     ):
         """
         link test - open up a connection to a specific provider, and send them an xml payload with a <msgType> of
@@ -234,16 +233,20 @@ class CBCProxyVodafone(CBCProxyClientBase):
     def send_link_test(
         self,
         identifier,
-        sequential_number,
     ):
         """
         link test - open up a connection to a specific provider, and send them an xml payload with a <msgType> of
         test.
         """
+        from app import db
+        sequence = Sequence('broadcast_provider_message_number_seq')
+        sequential_number = db.session.connection().execute(sequence)
+        formatted_seq_number = format_sequential_number(sequential_number)
+
         payload = {
             'message_type': 'test',
             'identifier': identifier,
-            'message_number': sequential_number,
+            'message_number': formatted_seq_number,
             'message_format': 'ibag'
         }