tests: Use TimestampWithTimezone.from_datetime() instead of the constructor
A future release of swh-model will change its constructor's signature (replace 'offset' and 'negative_utc_offset' with 'offset_bytes).
This leaves one occurence of a direct use of the constructor, as from_datetime() does not allow negative UTC.
Migrated from D6910 (view on Phabricator)