test: synchronize BigQuery write assertions

The BigQuery analytics plugin test relied on fixed 10ms sleeps to wait for asynchronous writes, which is not a valid completion signal and produced timing-dependent failures and leaked Queue.get coroutine warnings. This replaces the sleeps with the plugin's flush() synchronization boundary and shuts the loop-owned worker down before closing the event loop.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 951592599

Author: GWeale