Add a trivial test case for HeraldField

Summary: Adds a trivial test case to ensure that HeraldField subclasses are properly implemented.

Test Plan: arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13577