Prevent sporadic test failures

These tests occasionally fail because all the editions share the same
created_at timestamp, which means the order that they are returned in
by all_published_editions_in_creation_order is non-deterministic.
These changes to the test fixtures ensure the timestamps better
reflect when they should have been created.
Merge pull request #1336 from alphagov/ensure-created-at-timestamps-are-different

Prevent sporadic test failures