[Author: thorogood]

Adds [GTMTestCase imageFromResource:], as a helper method during tests.

R=thomasvl
APPROVED=thomasvl
DELTA=46  (44 added, 0 deleted, 2 changed)

git-svn-id: http://google-toolbox-for-mac.googlecode.com/svn/trunk@596 7dc7ac4e-7543-0410-b95c-c1676fc8e2a3