tests(utc): add unit tests for UTCDateTime and Postgres configured time zone

[palkerecsenyi]

Added unit tests to demonstrate how UTCDateTime behaves with all 4 scenarios:

• Pre-migration (timestamp without time zone columns) + DB is in UTC
  • Works as expected, no errors or value inconsistencies
• Pre-migration + DB is not in UTC
  • No error but times are incorrectly shifted
  • This is the worst-case scenario. The unit test demonstrates the problem. This is fixed in fix(config): use UTC for PostgreSQL #196 as long as instances don't override the config.
• Post-migration (timestamp with time zone columns) + DB is in UTC
  • Works as expected
• Post-migration + DB is not in UTC
  • Raises an error when reading values from the DB