Write tests for 'just turn up' message logic

[gestchild]

Related to https://github.com/wellcomecollection/wellcomecollection.org/pull/11164

Under certain circumstances we display a message on events within an event schedule.

We had to update this logic recently for an upcoming event and release it quickly.

It would be good to have tests for this logic so we are certain it functions as expected and can update it in future more confidently