Add a bit of test coverage for bulky vs compact edge data representations

Summary: Depends on D18948. Ref T13051. The actual logic ended up so simple that this doesn't really feel terribly valuable, but maybe it'll catch something later on.

Test Plan: Ran test.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13051

Differential Revision: https://secure.phabricator.com/D18949