Three improvements to the email-alert-api test helpers

[barrucadu]

Firstly, adding content_id to the subscriber lists URL helper.

The way the test helpers handle subscriber lists is kind of weird. We pass all the parameters---both the request and response ones undifferentiated---into a single has_subscriber_list method, which then uses an allowlist to pluck out the parameters which should go in the request URL, and uses the others to construct the response. This is pretty strange, the only benefit I can see is that some parameters you'd otherwise have to specify twice, but the cost is that we have this list here which has to be kept up to date. I think it would be better to remove the need for this entirely, but that can be a later change.

Secondly, adding sender_message_id and govuk_request_id to the bulk-unsubscribe bad-request helper.

These were left off because the only time the endpoint raises a bad request response is if you don't specify these. But that makes it kind of difficult to actually use in the email-alert-service tests: I could blank out the govuk_request_id easily enough, but I can't blank out the sender_message_id - so in practice this helper can't actually be used. So I've added those parameters in.

Thirdly, adding both with- and without-message versions of the bulk-unsubscribe helpers.

This is useful because, in cases where we don't care about what the message actually is (like the above case when we're testing a 422 response is gracefully handled), we can use the without-message variants.

[huwd]

Great improvements, nice one