Fix failing tests caused by Daylight Savings assumptions

1. EDT doesn't resolve to a timezone unless the tzinfo-data gem is loaded;
   use 'America/Chicago' instead, since the failure to resolve is transparent
   and results in the local clock's offset being used.
2. The test helper for local_offset needs to know about *when* we need an
   offset for, since they aren't consistent within named timezones over the
   course of the year.
3. Adds a verification to make sure offsets are being applied at all in the
   mocked time.