tests: Fix deprecation warning for utcfromtimestamp()

[ricardobranco777]

Fix DeprecationWarning for datetime.datetime.utcfromtimestamp()

It suggests to use timezone-aware objects to represent datetimes in UTC with datetime.UTC but datetime.timezone.utc is backwards compatible.

Verification run: https://openqa.opensuse.org/tests/5261910#downloads

tests/unit/api_container_test.py::ContainerTest::test_log_since_with_datetime
  /src/tests/unit/api_container_test.py:1305: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    time = datetime.datetime.utcfromtimestamp(ts)

tests/unit/api_test.py::DockerApiTest::test_events_with_since_until
  /src/tests/unit/api_test.py:234: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    now = datetime.datetime.utcfromtimestamp(ts)

[ricardobranco777]

Fixes deprecation warning noticed in #3335