Calendar: ICS: Unit tests

[benbucksch]

Create unit tests to check whether we can read the iCal/ICS files correctly and correctly interpret the data in it.

That means reading it in an Event object, writing it out as JSCalendar JSON object, and comparing the result with the expected result.
This should be based on test cases supplied by others, en masse.

Potential sources:

• https://github.com/jens-maus/node-ical/tree/master/test
• https://github.com/libical/libical/tree/master/test-data
• https://github.com/icalendar/icalendar/tree/main/spec/fixtures
• https://devguide.calconnect.org/Appendix/Examples/

Pay attention for licenses, please.

Other parsers or resources:

• https://git.sr.ht/~whynothugo/vparser
• https://github.com/CalConnect/DEVGUIDE-EXAMPLES/tree/master/

[fbartels]

Here are some more test sources:

• https://github.com/kewisch/ical.js/tree/main/samples
• https://github.com/peterbraden/ical.js/tree/master/test
• https://github.com/Kopano-dev/kopano-core/tree/master/libicalmapi/tests/ics
• https://github.com/Kopano-dev/kopano-core/tree/master/libicalmapi/tests/issues