Add unit tests for event parsing

The calendar allows users to add events using natural language processing see:
- Commit: https://github.com/elementary/calendar/commit/8a2063007acd73cbecf8d5c6877aafe20ee8b214
- [Add natural language parsing info #219](https://github.com/elementary/calendar/issues/219)

I'd like to improve it, but this is one of those prime examples where using unit tests would make that a lot easier.
Examples of unit tests in elementary:
- https://github.com/elementary/files/tree/master/libcore/tests/
- https://github.com/elementary/music/tree/master/core/Tests

But, adding tests [in Cmake](https://github.com/elementary/files/blob/master/libcore/tests/UtilTests/CMakeLists.txt) seems a lot harder than [in meson](https://github.com/elementary/music/blob/master/core/Tests/meson.build), so I think I'll wait for that to happen first.

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