Don't require the "gd" extension be installed in order to run unit tests

Summary:
Ref T10405. If gd is not installed, a number of unit tests currently fail because they generate synthetic users and try to composite profile pictures for them.

Instead, just fall back to a static default if gd is not available.

Test Plan: Faked this pathway, created a new user, saw a default profile picture.

Reviewers: amckinley, lpriestley, avivey

Reviewed By: avivey

Maniphest Tasks: T10405

Differential Revision: https://secure.phabricator.com/D18812