Add unit tests for event parsing

[peteruithoven]

The calendar allows users to add events using natural language processing see:

• Commit: 8a20630
• Add natural language parsing info #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 seems a lot harder than in meson, so I think I'll wait for that to happen first.

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[pantor]

When I wrote the basics of the event parsing stuff (a few years ago...), I had my own unit tests. They can still be found in this repository. I hope they are helpful 😉

[peteruithoven]

Thanks @pantor that's super helpful!