DRY-up logging and metrics for sending SMS

This avoids duplicating it as we add a new provider and means we
can test it all in one place (although it wasn't tested before).

I'm not sure why the previous code did "super(..)__init__" in a
non-init function - it's a bit late! - so I've just replaced it
with a call to the new "init_app" function in the parent class.

tests/app/clients/test_sms.py

@@ -0,0 +1,45 @@
+import pytest
+
+from app import statsd_client
+from app.clients.sms import SmsClient, SmsClientResponseException
+
+
+@pytest.fixture
+def fake_client(notify_api):
+    class FakeSmsClient(SmsClient):
+        @property
+        def name(self):
+            return 'fake'
+
+    fake_client = FakeSmsClient()
+    fake_client.init_app(notify_api, statsd_client)
+    return fake_client
+
+
+def test_send_sms(fake_client, mocker):
+    mock_send = mocker.patch.object(fake_client, 'try_send_sms')
+
+    fake_client.send_sms(
+        to='to',
+        content='content',
+        reference='reference',
+        international=False,
+    )
+
+    mock_send.assert_called_with(
+        'to', 'content', 'reference', False, None
+    )
+
+
+def test_send_sms_error(fake_client, mocker):
+    mocker.patch.object(
+        fake_client, 'try_send_sms', side_effect=SmsClientResponseException('error')
+    )
+
+    with pytest.raises(SmsClientResponseException):
+        fake_client.send_sms(
+            to='to',
+            content='content',
+            reference='reference',
+            international=False,
+        )