Calendar Import: add unit tests to cover participants

Summary:
Add unit tests to easily double-check matched participants in imported calendar events.

This will simplify the addition of future features without the risk to break older workflows.

Ref T15564

Closes T15905
Closes T15906

Test Plan:
See green lights over your new unit tests:

arc unit src/applications/calendar/import/__tests__/CalendarImportTestCase.php

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15906, T15564, T15905

Differential Revision: https://we.phorge.it/D25767