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