Add tests to show that get_or_create_event does not return closed events

[johannaengland]

When working on #258 I suddenly got worried that get_or_create_event might return an event after it has been closed (which is not the case since when closing an event we remove it from the index, which is what we use to get and event). But since I had that worry I decided to add a test just to document it a bit more.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.99%. Comparing base (b1ffc1c) to head (ea9a36e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #259 +/- ## ======================================= Coverage 98.99% 98.99% ======================================= Files 50 50 Lines 6740 6740 ======================================= Hits 6672 6672 Misses 68 68 ```

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