Add a test for sending email to provider

We had a test like this for sending sms, but not email. This meant that,
for example, we weren’t checking that the provider was getting passed
the HTML and plain text versions of the email.

tests/app/conftest.py

@@ -223,7 +223,7 @@ def sample_email_template_with_placeholders(notify_db, notify_db_session):
     return sample_email_template(
         notify_db,
         notify_db_session,
-        content="Hello ((name))",
+        content="Hello ((name))\nThis is an email from GOV.UK",
         subject_line="((name))")