Add test for alerts/notifications to test the snuba pipeline

[emmatyping]

This is part of #1761, should hopefully better test snuba.

[emmatyping]

We were advised that creating an alert would be a good way to test the snuba pipeline, however it seems that alert creation is a frontend only affair. The integration tests keep a cookie fresh, so perhaps the endpoint used by the frontend could still be used?

I'm also not sure how to handle alert firing, by default an email will be sent, I suppose we can check the server logs that happens.

[emmatyping]

I have been trying out playwright, with the idea that it could make testing alerts easier since the alert interface is rather UI focused. Unfortunately, it seems that the front end for Sentry does not have node identifiers, which makes playwright hard to integrate... back to curl/jq I suppose... :/