Add infrastructure to validate test images and a first test that does so.

cdms_test_4.png was generated on a ubuntu machine.
cdmf_test_4_1.png was generated on mac lion and reflect the bug in cairo.
When cairo is fixed, remove this version entirely.

This is a manual cherry pick to restore the test_validate branch that what
was in next before improve_build replaced next.
add an alternate test image until we can nail down the size
Fix test image size.

This size is what the dashboard produces. The difference between this and
interactive output appears to be cause by interaction with the window
manager.
Merge remote-tracking branch 'origin/new_test_validation'

* origin/new_test_validation:
  Fix test image size.
  add an alternate test image until we can nail down the size
  Add infrastructure to validate test images and a first test that does so.