Improve tests coverage of deleting expired events

[johannaengland]

Closes #209.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.04%. Comparing base (35f712c) to head (adfbe19).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #256 +/- ## ========================================== + Coverage 98.99% 99.04% +0.04% ========================================== Files 50 50 Lines 6739 6739 ========================================== + Hits 6671 6674 +3 + Misses 68 65 -3 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud