tests(streaming): use UTC timezone for start_timestamp and improve flaky retry delay

[shuoweil]

In tests/system/large/streaming/test_bigtable.py and tests/system/large/streaming/test_pubsub.py, continuous streaming queries are launched from BigQuery to Bigtable and Pub/Sub sinks.

1. start_timestamp was initialized with naive local datetime.now() - timedelta(days=1), which could cause timestamp offset mismatches across different CI runner timezones.
2. The retry delay was 10s, which is often insufficient for BigQuery continuous query slot capacity provisioning during retries under load.

Changes

• Updated start_timestamp to explicit UTC timezone (datetime.now(timezone.utc) - timedelta(days=1)).
• Increased @pytest.mark.flaky retry delay from 10s to 30s.

Fixes #<545715930> 🦕

[gemini-code-assist]

Code Review

This pull request updates the streaming tests for BigTable and PubSub by switching from naive local datetimes to timezone-aware UTC datetimes using datetime.now(timezone.utc). Additionally, it increases the retry delay for flaky tests from 10 seconds to 30 seconds to improve test stability. There are no review comments, and I have no feedback to provide.