chore(tests): fix timezone test

[daniel-sanche]

test_aggregation_query_get_stream_iterator_read_time_different_timezones currently fails when run in non-utc timezones

This section will build a different datetime value depending on the local machine's clock, since it's all based on a naive timestamp:

read_time = datetime(1970, 1, 1, 0, 30)
if timezone is not None:
    read_time = read_time.astimezone(timezone)

But the assertion assumes that the resulting timestamp value will be a fixed time in utc:

expected_timestamp = Timestamp(seconds=1800)

To fix this, I changed it to use datetime.fromtimestamp(1800, tz=custom_timezone)