Fix comparison of fixture timestamps across different timezones

Problem

This is a followup from !164 (merged). We changed the database timestamp columns to timestamp with time zone but we forgot to update the timestamps used in the fixtures. As a result, tests may fail (depending on the host timezone) when comparing database objects and fixtures.

Solution

Add a +00 suffix to the created_at and updated_at timestamps of fixtures.

Related to &2313 (closed).