utils: Timezone fixes and tests

- Update docstrings to explicitely mention Epoch
- Fix timezone bug in FormatTimestampWithTZ, where it would
  use GMT/UTC when it should use the local timezone
- Add unittests for time formatting functions

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>