Ensure fraction assets don't change
Ensure fraction assets don't change

Since these are now going to be depended on externally, we should make
sure nobody changes the contents of this directory.

If you came here because you are wondering why this test failed, you've
changed the fraction image assets and should update `hash` to match the
new fingerprint.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall