Add some working RRULE unit tests

Summary:
Ref T10747. These tests already work and didn't uncover any bugs.

(The next one deals with BYSETPOS and I think my approach isn't correct and needs significant work to correct, unfortuantely.)

Test Plan: Unit tests pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16613