Libraries used for freezing time in the sqlmesh unit tests.

Change motivated by freezegun's inability to handle timezones.

Notes:

• Default behavior
  • freezegun default is for time to be totally frozen inside the context, such that repeatedly capturing now() would give the same value every time
  • time_machine default is for time to continue inside the context
  • in the tests where it's relevant, we specify tick=False to retain freezegun's default behavior
• Time zones
  • time_machine infers tz as UTC for objects that are not tz-aware
  • If a tz-aware object is present without an explicit tz, the machine's local tz will be used
  • We manually specify UTC everywhere to avoid mixing tz-aware and non-tz-aware objects